Jekyll2020-03-28T14:31:50+00:00https://royf.org/feed/publications.xmlRoy Fox | PublicationsRoy Foxroy.d.fox@gmail.comHierarchical Variational Imitation Learning of Control Programs2019-12-29T00:00:00+00:002019-12-29T00:00:00+00:00https://royf.org/pub/Fox2019Hierarchical<p>Autonomous agents can learn by imitating teacher demonstrations of the intended behavior. Hierarchical control policies are ubiquitously useful for such learning, having the potential to break down structured tasks into simpler sub-tasks, thereby improving data efficiency and generalization. In this paper, we propose a variational inference method for imitation learning of a control policy represented by parametrized hierarchical procedures (PHP), a program-like structure in which procedures can invoke sub-procedures to perform sub-tasks. Our method discovers the hierarchical structure in a dataset of observation-action traces of teacher demonstrations, by learning an approximate posterior distribution over the latent sequence of procedure calls and terminations. Samples from this learned distribution then guide the training of the hierarchical control policy. We identify and demonstrate a novel benefit of variational inference in the context of hierarchical imitation learning: in decomposing the policy into simpler procedures, inference can leverage acausal information that is unused by other methods. Training PHP with variational inference outperforms LSTM baselines in terms of data efficiency and generalization, requiring less than half as much data to achieve a 24% error rate in executing the bubble sort algorithm, and to achieve no error in executing Karel programs.</p>Roy Foxroy.d.fox@gmail.comAutonomous agents can learn by imitating teacher demonstrations of the intended behavior. Hierarchical control policies are ubiquitously useful for such learning, having the potential to break down structured tasks into simpler sub-tasks, thereby improving data efficiency and generalization. In this paper, we propose a variational inference method for imitation learning of a control policy represented by parametrized hierarchical procedures (PHP), a program-like structure in which procedures can invoke sub-procedures to perform sub-tasks. Our method discovers the hierarchical structure in a dataset of observation-action traces of teacher demonstrations, by learning an approximate posterior distribution over the latent sequence of procedure calls and terminations. Samples from this learned distribution then guide the training of the hierarchical control policy. We identify and demonstrate a novel benefit of variational inference in the context of hierarchical imitation learning: in decomposing the policy into simpler procedures, inference can leverage acausal information that is unused by other methods. Training PHP with variational inference outperforms LSTM baselines in terms of data efficiency and generalization, requiring less than half as much data to achieve a 24% error rate in executing the bubble sort algorithm, and to achieve no error in executing Karel programs.Toward Provably Unbiased Temporal-Difference Value Estimation2019-12-14T00:00:00+00:002019-12-14T00:00:00+00:00https://royf.org/pub/Fox2019Toward<p>Temporal-difference learning algorithms, such as Q-learning, maintain and iteratively improve an estimate of the value that an agent can expect to gain in interaction with its environment. Unfortunately, the value updates in Q-learning induce positive bias that causes it to overestimate this value. Several algorithms, such as Soft Q-learning, regularize the value updates to reduce this bias, but none provide a principled schedule of their regularizers such that early in the learning process updates are more agnostic, but then increasingly trust the value estimates more as they become more certain during learning. In this paper, we present a closed-form expression for the regularization coefficient that completely eliminates bias in entropy-regularized value updates, and illustrate this theoretical analysis using a proof-of-concept algorithm that approximates the conditions for unbiased value estimation.</p>Roy Foxroy.d.fox@gmail.comTemporal-difference learning algorithms, such as Q-learning, maintain and iteratively improve an estimate of the value that an agent can expect to gain in interaction with its environment. Unfortunately, the value updates in Q-learning induce positive bias that causes it to overestimate this value. Several algorithms, such as Soft Q-learning, regularize the value updates to reduce this bias, but none provide a principled schedule of their regularizers such that early in the learning process updates are more agnostic, but then increasingly trust the value estimates more as they become more certain during learning. In this paper, we present a closed-form expression for the regularization coefficient that completely eliminates bias in entropy-regularized value updates, and illustrate this theoretical analysis using a proof-of-concept algorithm that approximates the conditions for unbiased value estimation.AutoPandas: Neural-Backed Generators for Program Synthesis2019-10-25T00:00:00+00:002019-10-25T00:00:00+00:00https://royf.org/pub/Bavishi2019AutoPandas<p>Developers nowadays have to contend with a growing number of APIs. While in the long-term they are very useful to developers, many modern APIs, with their hundreds of functions handling many arguments, obscure documentation, and frequently changing semantics, have an incredibly steep learning curve. For APIs that perform data transformations, novices can often provide an I/O example demonstrating the desired transformation, but are stuck on how to translate it to the API. A programming-by-example synthesis engine that takes such I/O examples and directly produces programs in the target API could help such novices. Such an engine presents unique challenges due to the breadth of real-world APIs, and the often-complex constraints over function arguments. We present a generator-based synthesis approach to contend with these problems. This approach uses a program candidate generator, which encodes basic constraints on the space of programs. We introduce neural-backed operators which can be seamlessly integrated into the program generator. To improve the efficiency of the search, we simply use these operators at non-deterministic decision points, instead of relying on domain-specific heuristics. We implement this technique for the Python pandas library in AutoPandas. AutoPandas supports 119 pandas dataframe transformation functions. We evaluate AutoPandas on 26 real-world benchmarks and find it solves 17 of them.</p>Roy Foxroy.d.fox@gmail.comDevelopers nowadays have to contend with a growing number of APIs. While in the long-term they are very useful to developers, many modern APIs, with their hundreds of functions handling many arguments, obscure documentation, and frequently changing semantics, have an incredibly steep learning curve. For APIs that perform data transformations, novices can often provide an I/O example demonstrating the desired transformation, but are stuck on how to translate it to the API. A programming-by-example synthesis engine that takes such I/O examples and directly produces programs in the target API could help such novices. Such an engine presents unique challenges due to the breadth of real-world APIs, and the often-complex constraints over function arguments. We present a generator-based synthesis approach to contend with these problems. This approach uses a program candidate generator, which encodes basic constraints on the space of programs. We introduce neural-backed operators which can be seamlessly integrated into the program generator. To improve the efficiency of the search, we simply use these operators at non-deterministic decision points, instead of relying on domain-specific heuristics. We implement this technique for the Python pandas library in AutoPandas. AutoPandas supports 119 pandas dataframe transformation functions. We evaluate AutoPandas on 26 real-world benchmarks and find it solves 17 of them.Multi-Task Hierarchical Imitation Learning for Home Automation2019-08-25T00:00:00+00:002019-08-25T00:00:00+00:00https://royf.org/pub/Fox2019Multi<p>Control policies for home automation robots can be learned from human demonstrations, and hierarchical control has the potential to reduce the required number of demonstrations. When learning multiple policies for related tasks, demonstrations can be reused between the tasks to further reduce the number of demonstrations needed to learn each new policy. We present HIL-MT, a framework for Multi-Task Hierarchical Imitation Learning, involving a human teacher, a networked Toyota HSR robot, and a cloud-based server that stores demonstrations and trains models. In our experiments, HIL-MT learns a policy for clearing a table of dishes from 11.2 demonstrations on average. Learning to set the table requires 19 new demonstrations when training separately, but only 11.6 new demonstrations when also reusing demonstrations of clearing the table. HIL-MT learns policies for building 3- and 4-level pyramids of glass cups from 8.2 and 5 demonstrations, respectively, but reusing the 3-level demonstrations for learning a 4-level policy only requires 2.7 new demonstrations. These results suggest that learning hierarchical policies for structured domestic tasks can reuse existing demonstrations of related tasks to reduce the need for new demonstrations.</p>Roy Foxroy.d.fox@gmail.comControl policies for home automation robots can be learned from human demonstrations, and hierarchical control has the potential to reduce the required number of demonstrations. When learning multiple policies for related tasks, demonstrations can be reused between the tasks to further reduce the number of demonstrations needed to learn each new policy. We present HIL-MT, a framework for Multi-Task Hierarchical Imitation Learning, involving a human teacher, a networked Toyota HSR robot, and a cloud-based server that stores demonstrations and trains models. In our experiments, HIL-MT learns a policy for clearing a table of dishes from 11.2 demonstrations on average. Learning to set the table requires 19 new demonstrations when training separately, but only 11.6 new demonstrations when also reusing demonstrations of clearing the table. HIL-MT learns policies for building 3- and 4-level pyramids of glass cups from 8.2 and 5 demonstrations, respectively, but reusing the 3-level demonstrations for learning a 4-level policy only requires 2.7 new demonstrations. These results suggest that learning hierarchical policies for structured domestic tasks can reuse existing demonstrations of related tasks to reduce the need for new demonstrations.Multi-Task Learning via Task Multi-Clustering2019-06-15T00:00:00+00:002019-06-15T00:00:00+00:00https://royf.org/pub/Yan2019Multi<p>Multi-task learning has the potential to facilitate learning of shared representations between tasks, leading to better task performance. Some sets of tasks are related, and can share many features that are useful latent representations for these tasks. Other sets of tasks are less related, possibly sharing some features, but also competing on the representational resources of shared parameters. We propose to discover how to share parameters between related tasks and split parameters between conflicting tasks, by learning a multi-clustering of the tasks. We present a mixture-of-experts model, where each cluster is an expert that extracts a feature vector from the input, and each task belongs to a set of clusters whose experts it can mix. In experiments on the CIFAR-100 MTL domain, multi-clustering outperforms a model that mixes all experts in accuracy and computation time. The results suggest that the performance of our method is robust to regularization that increases the model’s sparsity when sufficient data is available, and can benefit from sparser models as data becomes scarcer.</p>Roy Foxroy.d.fox@gmail.comMulti-task learning has the potential to facilitate learning of shared representations between tasks, leading to better task performance. Some sets of tasks are related, and can share many features that are useful latent representations for these tasks. Other sets of tasks are less related, possibly sharing some features, but also competing on the representational resources of shared parameters. We propose to discover how to share parameters between related tasks and split parameters between conflicting tasks, by learning a multi-clustering of the tasks. We present a mixture-of-experts model, where each cluster is an expert that extracts a feature vector from the input, and each task belongs to a set of clusters whose experts it can mix. In experiments on the CIFAR-100 MTL domain, multi-clustering outperforms a model that mixes all experts in accuracy and computation time. The results suggest that the performance of our method is robust to regularization that increases the model’s sparsity when sufficient data is available, and can benefit from sparser models as data becomes scarcer.Generalizing Robot Imitation Learning with Invariant Hidden Semi-Markov Models2018-12-09T00:00:00+00:002018-12-09T00:00:00+00:00https://royf.org/pub/Tanwani2018Generalizing<p>Generalizing manipulation skills to new situations requires extracting invariant patterns from demonstrations. For example, the robot needs to understand the demonstrations at a higher level while being invariant to the appearance of the objects, geometric aspects of objects such as its position, size, orientation and viewpoint of the observer in the demonstrations. In this paper, we propose an algorithm that learns a joint probability density function of the demonstrations with invariant formulations of hidden semi-Markov models to extract invariant segments (also termed as sub-goals or options), and smoothly follow the generated sequence of states with a linear quadratic tracking controller. The algorithm takes as input the demonstrations with respect to different coordinate systems describing virtual landmarks or objects of interest with a task-parameterized formulation, and adapt the segments according to the environmental changes in a systematic manner. We present variants of this algorithm in latent space with low-rank covariance decompositions, semi-tied covariances, and non-parametric online estimation of model parameters under small variance asymptotics; yielding considerably low sample and model complexity for acquiring new manipulation skills. The algorithm allows a Baxter robot to learn a pick-and-place task while avoiding a movable obstacle based on only 4 kinesthetic demonstrations.</p>Roy Foxroy.d.fox@gmail.comGeneralizing manipulation skills to new situations requires extracting invariant patterns from demonstrations. For example, the robot needs to understand the demonstrations at a higher level while being invariant to the appearance of the objects, geometric aspects of objects such as its position, size, orientation and viewpoint of the observer in the demonstrations. In this paper, we propose an algorithm that learns a joint probability density function of the demonstrations with invariant formulations of hidden semi-Markov models to extract invariant segments (also termed as sub-goals or options), and smoothly follow the generated sequence of states with a linear quadratic tracking controller. The algorithm takes as input the demonstrations with respect to different coordinate systems describing virtual landmarks or objects of interest with a task-parameterized formulation, and adapt the segments according to the environmental changes in a systematic manner. We present variants of this algorithm in latent space with low-rank covariance decompositions, semi-tied covariances, and non-parametric online estimation of model parameters under small variance asymptotics; yielding considerably low sample and model complexity for acquiring new manipulation skills. The algorithm allows a Baxter robot to learn a pick-and-place task while avoiding a movable obstacle based on only 4 kinesthetic demonstrations.Hierarchical Imitation Learning via Variational Inference of Control Programs2018-12-08T03:00:00+00:002018-12-08T03:00:00+00:00https://royf.org/pub/Fox2018Hierarchical<p>Autonomous controllers can be trained by imitation learning from demonstrations of the intended control. Hierarchical imitation learning in the parametrized hierarchical procedures (PHP) framework can reduce the required number of demonstrations by allowing each procedure to specialize in specific behavior and abstract away from transient state features. We propose a variational inference method for discovering the latent hierarchical structure in observation–action traces of teacher demonstrations. We train an inference model to approximate the posterior distribution of the latent call-stack of hierarchical procedures, and sample from it to guide the training of the hierarchical controller. Our method requires 40 demonstrations, less than half as many as end-to-end RNN training, to achieve 88% success rate in training the BubbleSort algorithm.</p>Roy Foxroy.d.fox@gmail.comAutonomous controllers can be trained by imitation learning from demonstrations of the intended control. Hierarchical imitation learning in the parametrized hierarchical procedures (PHP) framework can reduce the required number of demonstrations by allowing each procedure to specialize in specific behavior and abstract away from transient state features. We propose a variational inference method for discovering the latent hierarchical structure in observation–action traces of teacher demonstrations. We train an inference model to approximate the posterior distribution of the latent call-stack of hierarchical procedures, and sample from it to guide the training of the hierarchical controller. Our method requires 40 demonstrations, less than half as many as end-to-end RNN training, to achieve 88% success rate in training the BubbleSort algorithm.An Empirical Exploration of Gradient Correlations in Deep Learning2018-12-08T02:00:00+00:002018-12-08T02:00:00+00:00https://royf.org/pub/Rothchild2018Empirical<p>We introduce the mean and RMS dot product between normalized gradient vectors as tools for investigating the structure of loss functions and the trajectories followed by optimizers. We show that these quantities are sensitive to well-understood properties of the optimization algorithm, and we argue that investigating these quantities in detail can provide insight into properties that are less well understood. Using these tools, we observe that variance in the gradients of the loss function can be mostly explained by a small number of dimensions, and we compare results when training networks within the subspace spanned by the first few gradients to those obtained by training within a randomly chosen subspace.</p>Roy Foxroy.d.fox@gmail.comWe introduce the mean and RMS dot product between normalized gradient vectors as tools for investigating the structure of loss functions and the trajectories followed by optimizers. We show that these quantities are sensitive to well-understood properties of the optimization algorithm, and we argue that investigating these quantities in detail can provide insight into properties that are less well understood. Using these tools, we observe that variance in the gradients of the loss function can be mostly explained by a small number of dimensions, and we compare results when training networks within the subspace spanned by the first few gradients to those obtained by training within a randomly chosen subspace.Neural Inference of API Functions from Input–Output Examples2018-12-08T01:00:00+00:002018-12-08T01:00:00+00:00https://royf.org/pub/Bavishi2018Neural<p>Because of the prevalence of APIs in modern software development, an automated interactive code discovery system to help developers use these APIs would be extremely valuable. Program synthesis is a promising method to build such a system, but existing approaches focus on programs in domain-specific languages with much fewer functions than typically provided by an API. In this paper we focus on 112 functions from the Python pandas library for DataFrame manipulation, an order of magnitude more than considered in prior approaches. To assess the viability of program synthesis in this domain, our first goal is a system that reliably synthesizes programs with a single library function. We introduce an encoding of structured input–output examples as graphs that can be fed to existing graph-based neural networks to infer the library function. We evaluate the effectiveness of this approach on synthesized and real-world I/O examples, finding programs matching the I/O examples for 97% of both our validation set and cleaned test set.</p>Roy Foxroy.d.fox@gmail.comBecause of the prevalence of APIs in modern software development, an automated interactive code discovery system to help developers use these APIs would be extremely valuable. Program synthesis is a promising method to build such a system, but existing approaches focus on programs in domain-specific languages with much fewer functions than typically provided by an API. In this paper we focus on 112 functions from the Python pandas library for DataFrame manipulation, an order of magnitude more than considered in prior approaches. To assess the viability of program synthesis in this domain, our first goal is a system that reliably synthesizes programs with a single library function. We introduce an encoding of structured input–output examples as graphs that can be fed to existing graph-based neural networks to infer the library function. We evaluate the effectiveness of this approach on synthesized and real-world I/O examples, finding programs matching the I/O examples for 97% of both our validation set and cleaned test set.Constraint Estimation and Derivative-Free Recovery for Robot Learning from Demonstrations2018-08-22T00:00:00+00:002018-08-22T00:00:00+00:00https://royf.org/pub/Lee2018Constraint<p>Learning from human demonstrations can facilitate automation but is risky because the execution of the learned policy might lead to collisions and other failures. Adding explicit constraints to avoid unsafe states is generally not possible when the state representations are complex. Furthermore, enforcing these constraints during execution of the learned policy can be challenging in environments where dynamics are difficult to model such as push mechanics in grasping. In this paper, we propose Derivative-Free Recovery (DFR), a two-phase method for generating robust policies from demonstrations in robotic manipulation tasks where the system comes to rest at each time step. In the first phase, we use support estimation of supervisor demonstrations and treat the support as implicit constraints on states. We also propose a time-varying modification for sequential tasks. In the second phase, we use this support estimate to derive a switching policy that employs the learned policy in the interior of the support and switches to a recovery policy to steer the robot away from the boundary of the support if it drifts too close. We present additional conditions, which linearly bound the difference in state at each time step by the magnitude of control, allowing us to prove that the robot will not violate the constraints using the recovery policy. A simulated pushing task in MuJoCo suggests that DFR can reduce collisions by 83%. On a physical line tracking task using a da Vinci Surgical Robot and a moving Stewart platform, DFR reduced collisions by 84%.</p>Roy Foxroy.d.fox@gmail.comLearning from human demonstrations can facilitate automation but is risky because the execution of the learned policy might lead to collisions and other failures. Adding explicit constraints to avoid unsafe states is generally not possible when the state representations are complex. Furthermore, enforcing these constraints during execution of the learned policy can be challenging in environments where dynamics are difficult to model such as push mechanics in grasping. In this paper, we propose Derivative-Free Recovery (DFR), a two-phase method for generating robust policies from demonstrations in robotic manipulation tasks where the system comes to rest at each time step. In the first phase, we use support estimation of supervisor demonstrations and treat the support as implicit constraints on states. We also propose a time-varying modification for sequential tasks. In the second phase, we use this support estimate to derive a switching policy that employs the learned policy in the interior of the support and switches to a recovery policy to steer the robot away from the boundary of the support if it drifts too close. We present additional conditions, which linearly bound the difference in state at each time step by the magnitude of control, allowing us to prove that the robot will not violate the constraints using the recovery policy. A simulated pushing task in MuJoCo suggests that DFR can reduce collisions by 83%. On a physical line tracking task using a da Vinci Surgical Robot and a moving Stewart platform, DFR reduced collisions by 84%.