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Title: Unsupervised Control Through Non-Parametric Discriminative Rewards

Paper content:
Learning to control an environment without hand-crafted rewards or expert data remains challenging and is at the frontier of reinforcement learning research.
We present an unsupervised learning algorithm to train agents to achieve perceptually-specified goals using only a stream of observations and actions.
Our agent simultaneously learns a goal-conditioned policy and a goal achievement reward function that measures how similar a state is to the goal state.
This dual optimization leads to a co-operative game, giving rise to a learned reward function that reflects similarity in controllable aspects of the environment instead of distance in the space of observations.
We demonstrate the efficacy of our agent to learn, in an unsupervised manner, to reach a diverse set of goals on three domains -- Atari, the DeepMind Control Suite and DeepMind Lab.
Currently, the best performing methods on many reinforcement learning benchmark problems combine model-free reinforcement learning methods with policies represented using deep neural networks BID18 BID8 .
Despite reaching or surpassing human-level performance on many challenging tasks, deep model-free reinforcement learning methods that learn purely from the reward signal learn in a way that differs greatly from the manner in which humans learn.
In the case of learning to play a video game, a human player not only acquires a strategy for achieving a high score, but also gains a degree of mastery of the environment in the process.
Notably, a human player quickly learns which aspects of the environment are under their control as well as how to control them, as evidenced by their ability to rapidly adapt to novel reward functions BID22 .Focusing
learning on mastery of the environment instead of optimizing a single scalar reward function has many potential benefits. One benefit
is that learning is possible even in the absence of an extrinsic reward signal or with an extrinsic reward signal that is very sparse. Another benefit
is that an agent that has fully mastered its environment should be able to reach arbitrary achievable goals, which would allow it to generalize to tasks on which it wasn't explicitly trained. Building reinforcement
learning agents that aim for environment mastery instead of or in addition to learning about a scalar reward signal is currently an open challenge.One way to represent such knowledge about an environment is using an environment model. Modelbased reinforcement
learning methods aim to learn accurate environment models and use them either for planning or for training a policy. While learning accurate
environment models of some visually rich environments is now possible BID33 BID7 BID15 using learned models in model-based reinforcement learning has proved to be challenging and model-free approaches still dominate common benchmarks.We present a new model-free agent architecture of Discriminative Embedding Reward Networks, or DISCERN for short. DISCERN learns to control
an environment in an unsupervised way by learning purely from the stream of observations and actions. The aim of our agent is to
learn a goal-conditioned policy π θ (a|s; s g ) BID19 BID37 which can reach any goal state s g that is reachable from the current state s. We show how to learn a goal
achievement reward function r(s; s g ) that measures how similar state s is to state s g using a mutual information objective at the same time as learning π θ (a|s; s g ). The resulting learned reward
function r(s; s g ) measures similarity in the space of controllable aspects of the environment instead of in the space of raw observations. Crucially, the DISCERN architecture
is able to deal with goal states that are not perfectly reachable, for example, due to the presence of distractor objects that are not under the agent's control. In such cases the goal-conditioned
policy learned by DISCERN tends to seek states where the controllable elements match those in the goal state as closely as possible.We demonstrate the effectiveness of our approach on three domains -Atari games, continuous control tasks from the DeepMind Control Suite, and DeepMind Lab. We show that our agent learns to successfully
achieve a wide variety of visually-specified goals, discovering underlying degrees of controllability of an environment in a purely unsupervised manner and without access to an extrinsic reward signal.
We have presented a system that can learn to achieve goals, specified in the form of observations from the environment, in a purely unsupervised fashion, i.e. without any extrinsic rewards or expert demonstrations.
Integral to this system is a powerful and principled discriminative reward learning objective, which we have demonstrated can recover the dominant underlying degrees of controllability in a variety of visual domains.In this work, we have adopted a fixed episode length of T in the interest of simplicity and computational efficiency.
This implicitly assumes not only that all sampled goals are approximately achievable in T steps, but that the policy need not be concerned with finishing in less than the allotted number of steps.
Both of these limitations could be addressed by considering schemes for early termination based on the embedding, though care must be taken not to deleteriously impact training by terminating episodes too early based on a poorly trained reward embedding.
Relatedly, our goal selection strategy is agnostic to both the state of the environment at the commencement of the goal episode and the current skill profile of the policy, utilizing at most the content of the goal itself to drive the evolution of the goal buffer G. We view it as highly encouraging that learning proceeds using such a naive goal selection strategy, however more sophisticated strategies, such as tracking and sampling from the frontier of currently achievable goals BID16 , may yield substantial improvements.DISCERN's ability to automatically discover controllable aspects of the observation space is a highly desirable property in the pursuit of robust low-level control.
A natural next step is the incorporation of DISCERN into a deep hierarchical reinforcement learning setup BID46 BID24 BID30 where a meta-policy for proposing goals is learned after or in tandem with a low-level controller, i.e. by optimizing an extrinsic reward signal.

assistant: text: Unsupervised reinforcement learning method for learning a policy to robustly achieve perceptually specified goals.