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Title: Discriminator-Actor-Critic: Addressing Sample Inefficiency and Reward Bias in Adversarial Imitation Learning

Paper content:
We identify two issues with the family of algorithms based on the Adversarial Imitation Learning framework.
The first problem is implicit bias present in the reward functions used in these algorithms.
While these biases might work well for some environments, they can also lead to sub-optimal behavior in others.
Secondly, even though these algorithms can learn from few expert demonstrations, they require a prohibitively large number of interactions with the environment in order to imitate the expert for many real-world applications.
In order to address these issues, we propose a new algorithm called Discriminator-Actor-Critic that uses off-policy Reinforcement Learning to reduce policy-environment interaction sample complexity by an average factor of 10.
Furthermore, since our reward function is designed to be unbiased, we can apply our algorithm to many problems without making any task-specific adjustments.
The Adversarial Imitation Learning (AIL) class of algorithms learns a policy that robustly imitates an expert's actions via a collection of expert demonstrations, an adversarial discriminator and a reinforcement learning method.
For example, the Generative Adversarial Imitation Learning (GAIL) algorithm BID19 ) uses a discriminator reward and a policy gradient algorithm to imitate an expert RL policy.
Similarly, the Adversarial Inverse Reinforcement Learning (AIRL) algorithm BID10 ) makes use of a modified GAIL discriminator to recover a reward function to perform Inverse Reinforcement Learning (IRL) BID1 .
Additionally, this subsequent dense reward is robust to changes in dynamics or environment properties.
Importantly, AIL algorithms such as GAIL and AIRL, obtain higher performance than supervised Behavioral Cloning (BC) when using a small number of expert demonstrations; experimentally suggesting that AIL algorithms alleviate some of the distributional drift BID35 issues associated with BC.
However, these AIL methods suffer from two important issues that will be addressed by this work:
1) a large number of policy interactions with the learning environment is required for policy convergence and
2) although in principle these methods can learn rewards for absorbing states, the original implementations suffer from improper handling of the environment terminal states.
This introduces implicit rewards priors which can either improve or degrade policy performance.
Figure 1 : The Discriminator-Actor-Critic imitation learning framework combined with a method to explicitly learn rewards for the absorbing states.While GAIL requires as little as 200 expert frame transitions (from 4 expert trajectories) to learn a robust reward function on most MuJoCo BID41 tasks, the number of policy frame transitions sampled from the environment can be as high as 25 million in order to reach convergence.
If PPO ) is used in place of TRPO BID37 , the sample complexity can be improved (for example, as in Figure 3 , 25 million steps reduces to approximately 10 million steps), however it is still intractable for many robotics or real-world applications.
In this work we address this issue by incorporating an off-policy RL algorithm (TD3 BID11 ) and an off-policy discriminator to dramatically decrease the sample complexity by orders of magnitude.In this work, we also illustrate how specific design choices for AIL algorithms and MDPs used in practice, have a large impact on agent performance for environments with absorbing states.
For instance, as we will demonstrate, if the implementation assigns zero rewards for absorbing states, a strictly positive reward function can prevent the agent from solving tasks with a minimal number of steps, while a strictly negative reward function is unable to emulate a survival bonus.
Therefore, one must have some knowledge of the true environment reward and incorporate such priors to choose a suitable reward function for successful application of GAIL and AIRL.
We will discuss these issues formally, and present a simple -yet effective -solution that drastically improves policy performance for environments with absorbing states; we explicitly handle absorbing state transitions by learning the reward associated with these states.First we propose a new algorithm, which we call Discriminator-Actor-Critic (DAC) (Figure
1) , that is compatible with the GAIL and AIRL frameworks by extending them with an off-policy discriminator and an off-policy actor-critic reinforcement learning algorithm.
Then we propose a general approach to handling absorbing states in inverse reinforcement learning and reward learning methods.
We experimentally demonstrate that this removes the bias due to incorrect absorbing state handling in both GAIL-like and AIRL-like variants of our DAC algorithm.
In our experiments, we demonstrate that DAC achieves state-of-the-art AIL performance for a number of difficult imitation learning tasks, where proper handling of terminal states is crucial for matching expert performance in the presence of absorbing states.
More specifically, in this work we:• Identify, and propose solutions for the problem of handling terminal states of policy rollouts in standard RL benchmarks in the context of AIL algorithms.•
Accelerate learning from demonstrations by providing an off-policy variant for AIL algorithms, which significantly reduces the number of agent-environment interactions.•
Illustrate the robustness of DAC to noisy, multi-modal and constrained expert demonstrations, by performing experiments with human demonstrations on non-trivial robotic tasks.
In this work we address several important issues associated with the popular GAIL framework.
In particular, we address
1) sample inefficiency with respect to policy transitions in the environment and
2) we demonstrate a number of reward biases that can either implicitly impose prior knowledge about the true reward, or alternatively, prevent the policy from imitating the optimal expert.
To Figure 6 : Effect of learning absorbing state rewards when using an AIRL discriminator within the DAC Framework in OpenAI Gym environments.address reward bias, we propose a simple mechanism whereby the rewards for absorbing states are also learned, which negates the need to hand-craft a discriminator reward function for the properties of the task at hand.
In order to improve sample efficiency, we perform off-policy training of the discriminator and use an off-policy RL algorithm.
We show that our algorithm reaches state-of-theart performance for an imitation learning algorithm on several standard RL benchmarks, and is able to recover the expert policy given a significantly smaller number of samples than in recent GAIL work.

assistant: text: We address sample inefficiency and reward bias in adversarial imitation learning algorithms such as GAIL and AIRL.