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Workshop
Continuous Time Perspectives in Machine Learning
Mihaela Rosca · Chongli Qin · Julien Mairal · Marc Deisenroth

Sat Jul 23 06:00 AM -- 03:00 PM (PDT) @ Room 321 - 323

In machine learning, discrete time approaches such as gradient descent algorithms and discrete building layers for neural architectures have traditionally dominated. Recently, we have seen that by bridging these discrete systems with their continuous counterparts we can not only develop new insights but we can construct novel and competitive ML approaches. By leveraging time, we can tap into the centuries of research such as dynamical systems, numerical integration and differential equations, and continue enhancing what is possible in ML.The workshop aims to to disseminate knowledge about the use of continuous time methods in ML; to create a discussion forum and create a vibrant community around the topic; to provide a preview of what dynamical system methods might further bring to ML; to find the biggest hurdles in using continuous time systems in ML and steps to alleviate them; to showcase how continuous time methods can enable ML to have large impact in certain application domains, such as climate prediction and physical sciences.Recent work has shown that continuous time approaches can be useful in ML, but their applicability can be extended by increasing the visibility of these methods, fostering collaboration and an interdisciplinary approach to ensure their long-lasting impact. We thus encourage submissions with a varied set of topics: the intersection of machine learning and continuous-time methods; the incorporation of knowledge of continuous systems to analyse and improve on discrete approaches; the exploration of approaches from dynamical systems and related fields to machine learning; the software tools from the numerical analysis community.We have a diverse set of confirmed speakers and panellists with expertise in architectures, optimisation, RL, generative models, numerical analysis, gradient flows and climate. We hope this will foster an interdisciplinary and collaborative environment cohesive for the development of new research ideas.

 Sat 6:00 a.m. - 6:40 a.m. Deep neural network approximations for PDEs (Invited Talk)    Most of the numerical approximation methods for PDEs in the scientific literature suffer from the so-called curse of dimensionality (CoD) in the sense that the number of computational operations and/or the number of parameters employed in the corresponding approximation scheme grows exponentially in the PDE dimension and/or the reciprocal of the desired approximation precision. Recently, certain deep learning-based approximation methods for PDEs have been proposed and various numerical simulations for such methods suggest that deep neural network (DNN) approximations might have the capacity to indeed overcome the CoD in the sense that the number of real parameters used to describe the approximating DNNs grows at most polynomially in both the PDE dimension and the reciprocal of the prescribed approximation accuracy. In this talk, we show that solutions of suitable Kolmogorov PDEs can be approximated by DNNs without the CoD. Diyora Salimova 🔗 Sat 6:40 a.m. - 7:20 a.m. Reinforcement learning in continuous-time and space (Invited talk)    In this talk, we will introduce a continuous-time reinforcement learning (CTRL) framework. Our talk starts with a categorization of RL problems and naturally motivates a continuous-time perspective to RL. We then introduce a model-based CTRL approach, which solves physical control tasks using neural ordinary differential equations as a sub-routine. We conclude by briefly introducing recent approaches to CTRL. Cagatay Yildiz 🔗 Sat 7:20 a.m. - 7:40 a.m. Break 🔗 Sat 7:40 a.m. - 8:20 a.m. Generative Modeling with Stochastic Differential Equations (Invited talk)    Generative models are typically based on explicit representations of probability distributions (e.g., autoregressive or VAEs) or implicit sampling procedures (e.g., GANs). We propose an alternative approach based on modeling directly the vector field of gradients of the data distribution (scores). Our framework allows flexible architectures, requires no sampling during training or the use of adversarial training methods. Additionally, score-based generative models enable exact likelihood evaluation through connections with continuous time normalizing flows and stochastic differential equations. We produce samples comparable to GANs, achieving new state-of-the-art inception scores, and excellent likelihoods on image datasets. Stefano Ermon 🔗 Sat 8:20 a.m. - 8:30 a.m. Continuous-time event-based GRU for activity-sparse inference and learning (Contributed talk)    The scalability of recurrent neural networks (RNNs) is hindered by the sequential dependence of each time step’s computation on the previous time step’s output. Therefore, one way to speed up and scale RNNs is to reduce the computation required at each time step independent of model size and task. In this paper, we propose a time-continuous event-based model (EGRU) that extends Gated Recurrent Units (GRU) with an event-generation mechanism. This mechanism enforces activity-sparsity in time, and allows our model’s units to compute updates only on receipt of input events from other units. The combination of activity-sparsity and event-based computation has the potential to be computationally vastly more efficient than current RNNs. Notably, activity-sparsity in our model also translates into sparse parameter updates during gradient descent, extending this compute efficiency to the training phase. This sets the stage for the next generation of recurrent networks that are more scalable and efficient. Mark Schoene · Anand Subramoney · David Kappel · Khaleelulla Khan Nazeer · Christian Mayr 🔗 Sat 8:30 a.m. - 8:40 a.m. Irregularly-Sampled Time Series Modeling with Spline Networks (Contributed talk)    Observations made in continuous time are often irregular and contain the missing values across different channels. One approach to handle the missing data is imputing it using splines, by fitting the piecewise polynomials to the observed values. We propose using the splines as an input to a neural network, in particular, applying the transformations on the interpolating function directly, instead of sampling the points on a grid. To do that, we design the layers that can operate on splines and which are analogous to their discrete counterparts. This allows us to represent the irregular sequence compactly and use this representation in the downstream tasks such as classification and forecasting. Our model offers competitive performance compared to the existing methods both in terms of the accuracy and computation efficiency. Marin Biloš · Emanuel Ramneantu · Stephan Günnemann 🔗 Sat 8:40 a.m. - 8:50 a.m. Implicit Bias of Gradient Descent on Reparametrized Models: On Equivalence toMirror Descent (Contributed talk)    As part of the effort to understand implicit bias of gradient descent in overparametrized models, several results have shown how the training trajectory on the overparametrized model can be understood as mirror descent on a different objective. The main result here is a characterization of this phenomenon under a notion termed commuting parametrization, which encompasses all the previous results in this setting. It is shown that gradient flow with any commuting parametrization is equivalent to continuous mirror descent with a related mirror map. Conversely, continuous mirror descent with any mirror map can be viewed as gradient flow with a related commuting parametrization. The latter result relies upon Nash's embedding theorem. Zhiyuan Li · Tianhao Wang · Jason Lee · Sanjeev Arora 🔗 Sat 8:50 a.m. - 9:00 a.m. Heat Diffusion Based Recurrent Neural Differential Equations (Contributed talk)    Recurrent neural networks (RNN) are the primary choice for modelling sequential data, however they are less suitable for modelling irregular time-series data. Continuous time variants of RNN using neural ordinary differential equations (NODE) were shown to perform well on irregular time series data. They learn a better representation of the data using the continuous transformation of hidden states over time, taking into account the time interval between the observations. However, they are still limited in their capability as they use discrete number of layers (depth) over an input in the sequence to produce the output observation. We intend to address this limitation by proposing a RNN model designed based on the principle of heat equation. Our heat diffusion based recurrent neural differential equations(HDR-NDE) model generalizes RNN models by continuously evolving the hidden states in the temporal and depth dimension. HDR-NDE model is based on partial differential equations which treats the computation of hidden states as solving a heat equation over time. We demonstrate the effectiveness of the proposed model by comparing against the state-of-the-art RNN models on real world sequence modeling data sets. srinivas anumasa · geetakrishnasai gunapati · Srijith Prabhakaran nair kusumam 🔗 Sat 9:00 a.m. - 10:30 a.m. Lunch break (Break) 🔗 Sat 10:30 a.m. - 11:10 a.m. ResNet after all? How (not) to design continuous neural network architectures (Invited talk)    Can Neural ODE architectures provide a continuous-time extension of residual neural networks? I will show that this depends on the specific numerical solver chosen for training Neural ODE models. If the trained model is supposed to be a flow generated from an ODE, it should be possible to choose another numerical solver with equal or smaller numerical error without loss of performance. But if training relies on a solver with overly coarse discretization, then testing with another solver of equal or smaller numerical error results in a sharp drop in accuracy. In such cases, the combination of vector field and numerical method cannot be interpreted as a flow generated from an ODE, which arguably poses a fatal breakdown of the continuous-in-time concept. I will examine the specific effects which lead to this breakdown and discuss how to ensure that the model maintains continuous-time properties. Katharina Ott 🔗 Sat 11:10 a.m. - 11:50 a.m. Continuous vs. Discrete Optimization of Deep Neural Networks (Invited talk)    Existing analyses of optimization in deep learning are either continuous, focusing on variants of gradient flow (GF), or discrete, directly treating variants of gradient descent (GD). GF is amenable to theoretical analysis, but is stylized and disregards computational efficiency. The extent to which it represents GD is an open question in deep learning theory. My talk will present a recent study of this question. Viewing GD as an approximate numerical solution to the initial value problem of GF, I will show that the degree of approximation depends on the curvature around the GF trajectory, and that over deep neural networks (NNs) with homogeneous activations, GF trajectories enjoy favorable curvature, suggesting they are well approximated by GD. I will then use this finding to translate an analysis of GF over deep linear NNs into a guarantee that GD efficiently converges to global minimum almost surely under random initialization. Finally, I will present experiments suggesting that over simple deep NNs, GD with conventional step size is indeed close to GF. An underlying theme of the talk will be the possibility of GF (or modifications thereof) to unravel mysteries behind deep learning. Nadav Cohen 🔗 Sat 11:50 a.m. - 12:00 p.m. On the SDEs and Scaling Rules for Adaptive Gradient Algorithms (Contributed talk)    Approximating Stochastic Gradient Descent (SGD) as a Stochastic Differential Equation (SDE) has allowed researchers to enjoy the benefits of studying a continuous optimization trajectory while carefully preserving the stochasticity of SGD. Analogous study of adaptive gradient methods, such as RMSprop and Adam, has been challenging because there were no rigorously proven SDE approximations for these methods. This paper derives the SDE approximations for RMSprop and Adam, giving theoretical guarantees of their correctness as well as experimental validation of their applicability to common large-scaling vision and language settings. A key practical result is the derivation of a square root scaling rule to adjust the optimization hyperparameters of RMSprop and Adam when changing batch size, and its empirical validation in deep learning settings. Sadhika Malladi · Kaifeng Lyu · Abhishek Panigrahi · Sanjeev Arora 🔗 Sat 12:00 p.m. - 12:30 p.m. Tea Break (Break) 🔗 Sat 12:30 p.m. - 1:30 p.m. Panel (Discussion Panel)    A great panel discussion on continuous time methods in ML. Panel moderator: Michael N. Arbel, Research Fellow at the THOTH team of INRIA Grenoble. Panelists: Tatjana Chavdarova, Postdoctoral Fellow, UC Berkeley Ricky Chen, Research Scientist, Meta Priya Donti, PhD student, CMU Adil Salim, Research Scientist, Microsoft Research 🔗 Sat 1:30 p.m. - 3:00 p.m. Social and Poster session (Social and poster) 🔗 - Markovian Gaussian Process Autoencoders (Spotlight)    Deep generative models are widely used for modelling high-dimensional time series, such as video animations, audio and climate data. Sequential variational autoencoders have been successfully considered for many applications, with many variant models relying on discrete-time methods and recurrent neural networks (RNNs). On the other hand, continuous-time methods have recently gained attraction, especially in the context of irregularly-sampled time series, where they can better handle the data than discrete-time methods. One such class are Gaussian process variational autoencoders (GPVAEs), where the VAE prior is set as a Gaussian process (GPs), allowing inductive biases to be explicitly encoded via the kernel function and interpretability of the latent space. However, a major limitation of GPVAEs is that it inherits the same cubic computational cost as GPs. In this work, we leverage the equivalent discrete state space representation of Markovian GPs to enable a linear-time GP solver via Kalman filtering and smoothing. We show via corrupt and missing frames tasks that our method performs favourably, especially on the latter where it outperforms RNN-based models. Harrison Zhu · Carles Balsells Rodas · Yingzhen Li 🔗 - Contrasting Discrete and Continuous Time Methods for Bayesian System Identification (Spotlight)    In recent years, there has been considerable interest in embedding continuous time methods in machine learning algorithms. In system identification, the task is to learn a dynamical model from incomplete observation data, and when prior knowledge is in continuous time -- for example, mechanistic differential equation models -- it seems natural to use continuous time models for learning. Yet when learning flexible, nonlinear, probabilistic dynamics models, most previous work has focused on discrete time models to avoid computational, numerical, and mathematical difficulties. In this work we show, with the aid of small-scale examples, that this mismatch between model and data generating process can be consequential under certain circumstances, and we discuss possible modifications to discrete time models which may better suit them to handling data generated by continuous time processes. Talay Cheema · Carl E Rasmussen 🔗 - A Multistep Frank-Wolfe Method (Spotlight)    The Frank-Wolfe algorithm has regained much interest in its use in structurally constrained machine learning applications. However, one major limitation of the Frank-Wolfe algorithm is the slow local convergence property due to the zig-zagging behavior. We observe the zig-zagging phenomenon in the Frank-Wolfe method as an artifact of discretization, and propose multistep Frank-Wolfe variants where the truncation errors decay as $O(\Delta^p)$, where $p$ is the method's order. This strategy "stabilizes" the method, and allows tools like line search and momentum to have more benefit. However, our results suggest that the worst case convergence rate of Runge-Kutta-type discretization schemes cannot improve upon that of the vanilla Frank-Wolfe method for a rate depending on $k$. Still, we believe that this analysis adds to the growing knowledge of flow analysis for optimization methods, and is a cautionary tale on the ultimate usefulness of multistep methods. zhaoyue chen · Yifan Sun 🔗 - Everyone Matters: Customizing the Dynamics of Decision Boundary for Adversarial Robustness (Spotlight)    The adversarial robustness of a deep classifier can be measured by the robust radii: the decision boundary's distances to natural data points. However, it is unclear whether current adversarial training (AT) methods effectively improves the robust radius for each individual vulnerable point. To understand this, we propose a continuous-time framework that studies the relative speed of the decision boundary with respect to each individual point. Through visualizing the speed, a surprising conflicting moving-behavior is revealed: the decision boundary under AT moves away from some vulnerable points but simultaneously moves closer to other vulnerable ones. To alleviate this conflicting dynamics of the decision boundary, we propose Dynamical Customized Adversarial Training (Dyna-CAT) which directly controls the decision boundary to move away from the training data points. Moreover, in order to further encourage the robustness improvement for more vulnerable points, Dyna-CAT controls the decision boundary to move faster away from points with smaller robust radii, achieving customized manipulation of the decision boundary. As a result, Dyna-CAT achieves fairer robustness to individuals, leading to better overall robustness under limited model capacity. Experiments verify that Dyna-CAT alleviates the conflicting dynamics and obtains improved robustness compared with the state-of-the-art defenses. Yuancheng Xu · Yanchao Sun · Furong Huang 🔗 - Accelerated Methods for Distributed Optimization Problems using Fixed-time Stability of Continuous-time Dynamical Systems (Spotlight)    In this workshop paper, we present the recent developments on accelerated methods for solving constrained optimization problems using the notion of Fixed-time Stability (FxTS) utilizing the paradigm of continuous-time dynamical system. The notion of FxTS was first introduced in the field of control theory for studying fast convergence of trajectories of dynamical systems to their equilibrium point. We discuss how this concept can be used for optimization problems to solve them faster than the SOTA algorithms in distributed setting. Kunal Garg · Mayank Baranwal 🔗 - Faster Training of Neural ODEs Using Gauß–Legendre Quadrature (Spotlight)    Neural ODEs demonstrate strong performance in generative and time-series modelling. However, training them via the adjoint method is slow compared to discrete models due to the requirement of numerically solving ODEs. To speed neural ODEs up, a common approach is to regularise the solutions. However, this approach may affect the expressivity of the model; when the trajectory itself matters, this is particularly important. In this paper, we propose an alternative way to speed up the training of neural ODEs. The key idea is to speed up the adjoint method by using Gauß–Legendre quadrature to solve integrals faster than ODE-based methods while remaining memory efficient. Our approach leads to faster training of neural ODEs, especially for large models. Alexander Norcliffe · Marc Deisenroth 🔗 - Non-convex online learning via algorithmic equivalence (Spotlight)    We study an algorithmic equivalence technique between nonconvex gradient descent and convex mirror descent. We start by looking at a harder problem of regret minimization in online non-convex optimization. We show that under certain geometric and smoothness conditions, online gradient descent applied to non-convex functions is an approximation of online mirror descent applied to convex functions under reparameterization. In continuous time, the gradient flow with this reparameterization was shown to be exactly equivalent to continuous-time mirror descent by Amid and Warmuth, but theory for the analogous discrete time algorithms is left as an open problem. We prove an $O(T^{\frac{2}{3}})$ regret bound for non-convex online gradient descent in this setting, answering this open problem. Our analysis is based on a new and simple algorithmic equivalence method. Udaya Ghai · Zhou Lu · Elad Hazan 🔗 - Gradient Flows for L2 Support Vector Machine Training (Spotlight)    We explore the merits of training of support vector machines for binary classification by means of solving systems of ordinary differential equations. We thus assume a continuous time perspective on a machine learning problem which may be of interest for implementations on (re)emerging hardware platforms such as analog- or quantum computers. Christian Bauckhage · Rafet Sifa · Helen Schneider · Benjamin Wulff 🔗 - Recovering Stochastic Dynamics via Gaussian Schrödinger Bridges (Spotlight) We propose a new framework to reconstruct a stochastic process $\left\{\mathbb{P}_{t}: t \in[0, T]\right\}$ using only samples from its marginal distributions, observed at start and end times 0 and T. This reconstruction is useful to infer population dynamics, a crucial challenge, e.g., when modeling the time-evolution of cell populations from single-cell sequencing data. Our general framework encompasses the more specific Schrödinger bridge (SB) problem, where $\mathbb{P}_{t}$ represents the evolution of a thermodynamic system at almost equilibrium. Estimating such bridges from scratch is notoriously difficult, motivating our proposal for a novel adaptive scheme called the GSBflow. Our approach is to first perform a Gaussian approximation of the general SB via matching the moments of the data, which proves to significantly stabilize the training of SB. To that end, we solve the SB problem with Gaussian marginals, for which we provide, as a central contribution, a closed-form solution, and SDE representation. We use these formulas to define the reference process used to estimate more complex SBs, and obtain notable numerical improvements when reconstructing both synthetic processes and single-cell genomics. Ya-Ping Hsieh · Charlotte Bunne · Marco Cuturi · Andreas Krause 🔗 - Modeling Solutions to Ordinary and Partial Differential Equations with Continuous Initial Value Networks (Spotlight)    Differential equations play an important role in many different domains as they are used to describe the change in various real world systems. Previous works combined neural networks with differential equations to specify the dynamic or learn the solution. In this paper, we propose a general framework for modeling the solutions to ordinary and partial differential equations which relies on satisfying certain requirements so that the learned model always corresponds to the solution of the target equation. In particular, we propose novel flow models based on an efficient matrix exponential transformation to model ODE solutions. We extend this to stochastic differential equations and discuss suitable training strategies. Finally, we design models that are solutions to PDEs while respecting the initial and boundary conditions. Our models can be used in physics-informed learning, as well as to learn the mappings between the function spaces by defining a neural operator. Throughout the experiments, we demonstrate the benefits of using our method both in terms of predictive and computational performance. Marin Biloš · Andrei Smirdin · Stephan Günnemann 🔗 - Epsilon-Greedy Reinforcement Learning Policy in Continuous-Time Systems (Spotlight)    This work studies theoretical performance guarantees of a ubiquitous reinforcement learning policy for a canonicalcontinuous-time model. We show that epsilon-Greedy addresses the exploration-exploitation dilemma forminimizing quadratic costs in linear dynamical systems that evolve according to stochastic differential equations.More precisely, we establish square-root of time regret bounds, indicating that epsilon-Greedy learns optimalcontrol actions fast from a single state trajectory. Further, linear scaling of the regret with the number of parametersis shown. The presented analysis introduces novel and useful technical approaches, and sheds light on fundamentalchallenges of continuous-time reinforcement learning. Mohamad Kazem Shirani Faradonbeh 🔗 - Temporal Graph Neural Networks with Time-Continuous Latent States (Spotlight)    We propose a temporal graph neural network model for graph-structured irregular time series. The model is designed to handle both irregular time steps and partial graph observations. This is achieved by introducing a time-continuous latent state in each node of the graph. The latent dynamics are defined using a state-dependent decay-mechanism. Observations in the graph neighborhood are taken into account by integrating graph neural network layers in both the state update and predictive model. Experiments on a traffic forecasting task validate the usefulness of both the graph structure and time-continuous dynamics in this setting. Joel Oskarsson · Per Sidén · Fredrik Lindsten 🔗 - Continuous Methods : Adaptively intrusive reduced order model closure (Spotlight)    Reduced order modeling methods are often used as a means to reduce simulation costs in industrial applications. Despite their computational advantages, reduced order models (ROMs) often fail to accurately reproduce complex dynamics encountered in real life applications. To address this challenge, we leverage NeuralODEs to propose a novel ROM correction approach based on a time-continuous memory formulation. Finally, experimental results show that our proposed method provides a high level of accuracy while retaining the low computational costs inherent to reduced models. Emmanuel Menier · Michele Alessandro Bucci · Mouadh Yagoubi · Lionel Mathelin · Raphael Meunier · Thibault Dairay · Marc Schoenauer 🔗 - Continuous Methods : Hamiltonian Domain Translation (Spotlight)    This paper proposes a novel approach to domain translation. Leveraging established parallels between generative models and dynamical systems, we propose a reformulation of the Cycle-GAN architecture. By embedding our model with a Hamiltonian structure, we obtain a continuous, expressive and most importantly invertible generative model for domain translation. Emmanuel Menier · Michele Alessandro Bucci · Mouadh Yagoubi · Lionel Mathelin · Marc Schoenauer 🔗 - When Neural ODE Meets Adaptive Moment Estimation: Boosting Efficiency, Stability and Accuracy of Neural ODEs Together (Spotlight)    Recent work by Xia et al. leveraged the continuous-limit of the classical momentum accelerated gradient descent and proposed heavy-ball neural ODEs. While this model offers computational efficiency and high utility over vanilla neural ODEs, this approach often causes the overshooting of internal dynamics, leading to unstable training of a model. Prior work addresses this issue by using ad-hoc approaches, e.g., bounding the internal dynamics using specific activation functions, but the resulting models do not satisfy the exact heavy-ball ODE. In this work, we propose adaptive momentum estimation neural ODEs (AdamNODEs) that adaptively control the acceleration of the classical momentum-based approach. We find that We find that its adjoint states also satisfy AdamODE and do not require ad-hoc solutions that the prior work employs. In evaluation, we show that AdamNODEs achieve the lowest training loss and efficacy over existing neural ODEs. We also show that AdamNODEs have better training stability than classical momentum-based neural ODEs. This result sheds some light on adapting the techniques proposed in the optimization community to improving the training and inference of neural ODEs further. Seunghyeon Cho · Sanghyun Hong · Kookjin Lee · Noseong Park 🔗 - Two-Timescale Stochastic Approximation for Bilevel Optimisation Problems in Continuous-Time Models (Spotlight)    We analyse the asymptotic properties of a continuous-time, two-timescale stochastic approximation algorithm designed for stochastic bilevel optimisation problems in continuous-time models. We obtain the weak convergence rate of this algorithm in the form of a central limit theorem. We also demonstrate how this algorithm can be applied to several continuous-time bilevel optimisation problems. Louis Sharrock 🔗 - A New Look on Diffusion Times for Score-based Generative Models (Spotlight)    Score-based diffusion models map noise into data using stochastic differential equations. While current practice advocates for a large $T$ to ensure closeness to steady state, a smaller value of $T$ should be preferred for a better approximation of the score-matching objective and computational efficiency. We conjecture, contrary to current belief and corroborated by numerical evidence, that the optimal diffusion times are smaller than current practice. Giulio Franzese · Simone Rossi · Lixuan YANG · alessandro finamore · Dario Rossi · Maurizio Filippone · Pietro Michiardi 🔗 - Towards a General Purpose CNN for Long Range Dependencies in $N$D (Spotlight)    The use of Convolutional Neural Networks (CNNs) is widespread in Deep Learning due to a range of desirable model properties which result in an efficient and effective machine learning framework. However, performant CNN architectures must be tailored to specific tasks in order to incorporate considerations such as the input length, resolution, and dimentionality. In this work, we overcome the need for problem-specific CNN architectures with our Continuous Convolutional Neural Network (CCNN): a single CNN architecture equipped with continuous convolutional kernels that can be used for tasks on data of arbitrary resolution, dimensionality and length without structural changes. Continuous convolutional kernels model long range dependencies at every layer, and remove the need for downsampling layers and task-dependent depths needed in current CNN architectures. We show the generality of our approach by applying the same CCNN to a wide set of tasks on sequential ($1D$) and visual data ($2D$). Our CCNN performs competitively and often outperforms the current state-of-the-art across all tasks considered. David Romero · David Knigge · Albert Gu · Erik Bekkers · Efstratios Gavves · Jakub Tomczak · Mark Hoogendoorn 🔗 - Learning to Discretize for Continuous-time Sequence Compression (Spotlight) Neural compression offers a domain-agnostic approach to creating codecs for lossy or lossless compression via deep generative models. For sequence compression, however, most deep sequence models have costs that scale with the sequence length rather than the sequence complexity. In this work, we instead treat data sequences as observations from an underlying continuous-time process and learn how to efficiently discretize while retaining information about the full sequence. As a consequence of decoupling sequential information from its temporal discretization, our approach allows for greater compression rates and smaller computational complexity. Moreover, the continuous-time approach naturally allows us to decode at different time intervals and is amenable to randomly missing data, an important property for streaming applications. We empirically verify our approach on multiple domains involving compression of video and motion capture sequences, showing that our approaches can automatically achieve significant reductions in bit rates. Ricky T. Q. Chen · Maximilian Nickel · Matthew Le · Matthew Muckley · Karen Ullrich 🔗 - The Gap Between Continuous and Discrete Gradient Descent (Spotlight)    While it is possible to obtain valuable insights by analyzing gradient descent (GD) in its continuous form, we argue that a complete understanding of the mechanics leading to GD's success may indeed require considering effects of using a large step size in the discrete regime. To support this claim, we demonstrate the difference in trajectories for small and large learning rates when GD is applied on a neural network, observing effects of an escape from a local minimum with a large step size. Furthermore, it has been widely observed in neural network training that when applying stochastic gradient descent (SGD), a large step size is essential for obtaining superior models. In this work, through a novel set of experiments, we show even though stochastic noise is beneficial, it is not enough to explain success of SGD and a large learning rate is essential for obtaining the best performance even in stochastic settings. Finally, we prove on a certain class of functions that GD with large step size follows a different trajectory than GD with a small step size which can facilitate convergence to the global minimum. Amirkeivan Mohtashami · Martin Jaggi · Sebastian Stich 🔗 - Principle of Least Action Approach to Accelerate Neural Ordinary Differential Equations (Spotlight)    Neural ordinary differential equations(NODE) generalize discrete ResNet models by continuously transforming the hidden representations. NODE treats the computation of hidden states as computing the trajectory of an ordinary differential equation(ODE) parameterized by a neural network, which is expensive in terms of number of function evaluations. In this work, we propose a regularisation technique to decrease the number of function evaluations which is built on the framework of principle of least action (PLA) . In dynamics, the path chosen by an object to move from from one point to another is such that the action is minimum. Action is defined as the integral of the Lagrangian along the path. In our proposed approach, the trajectory computed by the NODE is controlled by a regularizer will be analogues to minimizing the action. We experimentally show that our proposed regularizer indeed requires less number of function evaluations. srinivas anumasa · Srijith Prabhakaran nair kusumam 🔗 - Estimating Treatment Effects in Continuous Time with Hidden Confounders (Spotlight)    Estimating individual treatment effects (ITEs) plays a crucial role in many real-world applications involving policy analysis and decision making. Nevertheless, estimating treatment effects in the longitudinal setting in the presence of hidden confounders remains an extremely challenging problem. Recently, there is a growing body of work attempting to obtain unbiased ITE estimates from time-dynamic observational data by ignoring the possible existence of hidden confounders. Additionally, many existing works handling hidden confounders are not applicable for continuous-time settings.In this paper, we extend the line of work focusing on deconfounding in the dynamic time setting in the presence of hidden confounders. We leverage recent advancements in neural differential equations to build a latent factor model using a stochastic controlled differential equation and Lipschitz constrained convolutional operation in order to continuously incorporate information about ongoing interventions and irregularly sampled observations. Experiments on both synthetic and real-world datasets highlight the promise of continuous time methods for estimating treatment effects in the presence of hidden confounders. Defu Cao · James Enouen · Yan Liu 🔗 - Continuous-time Analysis for Variational Inequalities: An Overview & Desiderata (Spotlight)    The optimization of zero-sum games, multi-objective agent training, or in general, the optimization of variational inequality (VI) problems is currently notoriously unstable on general problems. Owing to the increased need for training such models in machine learning, the above observation attracted significant research attention over the past years. Substantial progress has been made towards understanding the qualitative differences with single-objective minimization by casting the optimization method in its corresponding continuous-time dynamics, as well as obtaining convergence guarantees and rates for some instances of VIs because such guarantees often guide the corresponding proof for the discrete counterpart. Most notably, continuous-time tools allowed for analyzing complex non-convex problems, which in some cases, cannot be carried out using standard discrete-time tools. This paper aims to provide an overview of these ideas specifically for the broad VI problem class, and the insights originating from applying continuous-time tools for VI problems. We finalize by describing various desiderata of fundamental open questions towards developing optimization methods that work for general VIs and argue that tackling these requires understanding the associated continuous-time dynamics. Tatjana Chavdarova · Ya-Ping Hsieh 🔗 - MQTransformer: Context Dependent Attention and Bregman Volatility (Spotlight)    In many forecasting applications (e.g. retail demand, electricity load, weather, finance, etc.), the forecasts must obey certain properties such as having certain context-dependent and time-varying seasonality patterns and avoiding excessive revision as new information becomes available. Here we propose a new forecasting neural net architecture that addresses some of these issues, MQ-Transformer, by incorporating three architectural improvements to the current state-of-the-art: 1) a novel decoder-encoder attention that aligns the historical and future time periods 2) a novel positional encoding that learns seasonality from the historical time series and 3) a novel decoder-self attention that allows the network to minimize the forecast volatility. We then define a new measure of forecast volatility, Bregman Volatility, to understand one major source of the improvement from our model. Bregman Volatility allows us to compute the optimal volatility of a sequence of forecasts in terms of the improvement in forecast accuracy over that time period. We show both theoretically and empirically that the decoder-self attention module optimizes Bregman volatility and thereby improves forecast accuracy as well. Carson Eisenach · Dhruv Madeka · Kevin Chen · Lee Dicker 🔗 - Physics-Informed Neural Operator for Learning Partial Differential Equations (Spotlight) Machine learning methods have recently shown promise in solving partial differential equations (PDEs). They can be classified into two broad categories: approximating the solution function and learning the solution operator. The Physics-Informed Neural Network (PINN) is an example of the former while the Fourier neural operator (FNO) is an example of the latter. Both these approaches have shortcomings. The optimization in PINN is challenging and prone to failure, especially on multi-scale dynamic systems. FNO does not suffer from this optimization issue since it carries out supervised learning on a given dataset, but obtaining such data may be too expensive or infeasible. In this work, we propose the physics-informed neural operator (PINO), where we combine the operating-learning and function-optimization frameworks. This integrated approach improves convergence rates and accuracy over both PINN and FNO models. In the operator-learning phase, PINO learns the solution operator over multiple instances of the parametric PDE family. In the test-time optimization phase, PINO optimizes the pre-trained operator ansatz for the querying instance of the PDE. Experiments show PINO outperforms previous ML methods on many popular PDE families while retaining the extraordinary speed-up of FNO compared to solvers. In particular, PINO accurately solves long temporal transient flows and Kolmogorov flows where other baseline methods fail to converge. Zongyi Li · Hongkai Zheng · Nikola Kovachki · David Jin · Haoxuan Chen · Burigede Liu · Kamyar Azizzadenesheli · Animashree Anandkumar 🔗 - Riemannian Diffusion Schr\"odinger Bridge (Spotlight)    Score-based generative models exhibits state of art performance on density estimation and generative modeling tasks.These models typically assume that the data geometry is flat, yet recent extensions have been developed to model data living on Riemannian manifolds. Existing methods to accelerate sampling of diffusion models are typically not applicable in the Riemannian setting and Riemannian score-based methods have not yet been adapted to the important task of interpolation of datasets. To overcome these issues, we introduce \emph{Riemannian Diffusion Schr\"odinger Bridge} (RDSB).Our proposed method generalizes Diffusion Schr\"odinger Bridge introduced in \cite{debortoli2021neurips} to the non-Euclidean setting and as such generalizes Riemannian score-based models beyond the first time reversal. We validate our proposed method on synthetic data and real Earth and climate data. James Thornton · Valentin De Bortoli · Michael Hutchinson · Emile Mathieu · Yee Whye Teh · Arnaud Doucet 🔗 - Data Assimilation and Neural ODEs for learning latent dynamics (Spotlight)    The development of data-informed predictive models for dynamical systems is of widespread interest in many disciplines.We present a unifying framework for blending mechanistic and machine-learning approachesto identify dynamical systems from noisily and partially observed time-series data.Our formulation is agnostic to the chosen machine learning model,is presented in both continuous- and discrete-time settings,and is compatible both with systems that exhibit substantial memory and systems that are memoryless.We conclude with a series of numerical results thata) illustrate trade-offs when learning dynamics in continuous- and discrete-time,and b) demonstrate the inference power of our methodology in a partially observed Lorenz '63 system. Matthew Levine · Andrew Stuart 🔗 - Connections between Kernel Analog Forecasting and Gaussian Process Regression (Spotlight)    In this short communication we expose connections between two data-driven machine learning methods, kernel analog forecasting (KAF) and Gaussian process regression (GPR). In particular, it is shown that there are three major points in which KAF differs from GPR: the use of a specific kernel, normalization that guarantees spectrum to lie in $(0, 1]$, and spectral truncation, which acts both as a computational speed-up and regularization. Dmitry Burov 🔗 - Identification of Hidden Clusters of Time Series with Hybrid Neural Networks Integrating Expert Models (Spotlight)    Deep learning-based approaches for time series analysis notoriously suffer from interpretability and robustness issues due to their black-box nature. In this work, we propose a hybrid neural network model with embedded expert knowledge. We assume the time series are generated by a finite set of dynamics with known functional form. Our experiments show that our approach is more interpretable, and better at reconstruction than its black-box counterparts. András Formanek · Edward De Brouwer · Péter Antal · Yves Moreau · Adam Arany 🔗 - Should You Follow the Gradient Flow? Insights from Runge-Kutta Gradient Descent (Spotlight)    Recently, it has become popular in the machine learning community to model gradient-based optimization algorithms as ordinary differential equations (ODEs). Moreover, state-of-the-art optimizers such as SGD and Momentum can be recovered from the corresponding ODE using first-order numerical integrators such as explicit and symplectic Euler methods. In contrast, very little theoretical and experimental investigation has been carried out on the properties of higher-order integrators in optimization. In this paper, we analyze the properties of high-order Runge-Kutta (RK) integrators on gradient flows, in the context of both convex optimization and deep learning. We show that, while RK provides a close approximation to the gradient flow, this induces an increase in sharpness (maximum Hessian eigenvalue) at the solution – a feature which is believed to be negatively correlated with generalization. In addition, we show that, while high-order RK descent methods are stable for a broad range of stepsizes, convergence speed (in terms of training loss) is usually negatively affected by the method order. Xiang Li · Antonio Orvieto 🔗