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Title: Learning Temporal Abstraction with Information-theoretic Constraints for Hierarchical Reinforcement Learning

Paper content:
Applying reinforcement learning (RL) to real-world problems will require reasoning about action-reward correlation over long time horizons.
Hierarchical reinforcement learning (HRL) methods handle this by dividing the task into hierarchies, often with hand-tuned network structure or pre-defined subgoals.
We propose a novel HRL framework TAIC, which learns the temporal abstraction from past experience or expert demonstrations without task-specific knowledge.
We formulate the temporal abstraction problem as learning latent representations of action sequences and present a novel approach of regularizing the latent space by adding information-theoretic constraints.
Specifically, we maximize the mutual information between the latent variables and the state changes.
A visualization of the latent space demonstrates that our algorithm learns an effective abstraction of the long action sequences.
The learned abstraction allows us to learn new tasks on higher level more efficiently.
We convey a significant speedup in convergence over benchmark learning problems.
These results demonstrate that learning temporal abstractions is an effective technique in increasing the convergence rate and sample efficiency of RL algorithms.
Reinforcement learning (RL) has been successfully applied to many different tasks (Mnih et al., 2015; Zhu et al., 2017) .
However, applying it to real-world tasks remains a challenging problem, mainly due to the large search space and sparse reward signals.
In order to solve this, many research efforts have been focused on the hierarchical reinforcement learning (HRL), which decomposes an RL problem into sub-goals.
By solving the sub-goals, low-level actions are composed into high-level temporal abstractions.
In this way, the size of the searching space is decreased exponentially.
However, the HRL often requires explicitly specifying task structures or sub-goals (Barto & Mahadevan, 2003; Arulkumaran et al., 2017) .
How to learn those task structures or temporal abstractions automatically is still an active studying area.
Many different strategies are proposed for automatically discovering the task hierarchy or learning the temporal abstraction.
Some early studies try to find sub-goals or critical states based on statistic methods (Hengst, 2002; Jonsson, 2006; Kheradmandian & Rahmati, 2009 ).
More recent work seeks to learn the temporal abstraction with deep learning (Florensa et al., 2017; Tessler et al., 2017; Haarnoja et al., 2018a) .
However, many of these methods still require a predefined hierarchical policy structure (e.g. the number of sub-policies), or need some degree of task-specific knowledge (e.g. hand-crafted reward function).
We present a general HRL framework TAIC (Temporal Abstraction with Information-theoretic Constraints), which allows an agent to learn the temporal abstraction from past experiences or expert demonstrations without task-specific knowledge.
Built upon the ideas of options framework (Sutton et al., 1999) and motor skills (Lin, 1993) , we formulate the temporal abstraction problem as learning a latent representation of action sequences.
In order to obtain good latent representations, we propose a novel approach to regularize the latent space by using information-theoretic constraints.
The learned abstract representations of action sequences (we called options) allow us to do RL at a higher level, and easily transfer the knowledge between different tasks.
Our contributions are: 1) We formulate the temporal abstraction problem as learning a latent representation of action sequences.
Motivated by works using Recurrent Variational AutoEncoders (RVAE) to model sequential data in neural language processing (NLP) and other areas (Bowman et al., 2015; Ha & Eck, 2017) , we employ RVAE to perform temporal abstraction in RL.
2) We propose a regularization approach on the option space.
It constrains the option to encode more information about its consequence (how the option changes the states).
We present both theoretical derivations and practical solutions.
3) We show in the experiments that our learned temporal abstraction conveys meaningful information and benefit the RL training.
In addition, the proposed framework provides an efficient tool for transferring knowledge between tasks.
This paper presented a general HRL framework TAIC for learning temporal abstraction from action sequences.
We formulate the temporal abstraction problem as learning latent representations (called options) over action sequences.
In order to learn a better representation, we derive theoretically on how to regularize the option space and give an applicable solution of adding constraints to option space.
In the experiments, we try to reveal the underlying structure of the option space by visualizing the correlation between options and state changes.
We showed qualitatively and quantitatively that our options encode meaningful information and benefit the RL training.
Furthermore, the TAIC framework provides an efficient tool to transfer the knowledge learned from one task to another.
Our framework can be applied together with all kinds of RL optimization algorithms, and can be applied to both discrete and continuous problems.
This work brings many new directions for future studies.
As we currently learn the RL task and the option separately, the option could not be improved with the improvement of the policy.
In theory, it is entirely feasible to jointly optimize the two parts, or at least train them alternately.
As mentioned above, the current sub-policy acts like an open-loop controller.
So learning a close-loop sub-policy beyond the RNN decoder will be one of the focus areas of our future studies.
We would also like to apply the TAIC framework to discrete problems and with other RL algorithms such as DQN and SAC.
This could bring more insights to further improve the framework.

assistant: text: We propose a novel HRL framework, in which we formulate the temporal abstraction problem as learning a latent representation  of  action  sequence.