Paper:
Published as a conference paper at ICLR 2018 META LEARNING SHARED HIERARCHIES Work done as an intern at OpenAI
We develop a metalearning approach for learning hierarchically structured policies, improving sample efficiency on unseen tasks through the use of shared primitives-policies that are executed for large numbers of timesteps. Specifically, a set of primitives are shared within a distribution of tasks, and are switched between by task-specific policies. We provide a concrete metric for measuring the strength of such hierarchies, leading to an optimization problem for quickly reaching high reward on unseen tasks. We then present an algorithm to solve this problem end-to-end through the use of any off-the-shelf reinforcement learning method, by repeatedly sampling new tasks and resetting task-specific policies. We successfully discover 1 meaningful motor primitives for the directional movement of four-legged robots, solely by interacting with distributions of mazes. We also demonstrate the transferability of primitives to solve long-timescale sparse-reward obstacle courses, and we enable 3D humanoid robots to robustly walk and crawl with the same policy.

Review:
This paper proposes a novel hierarchical reinforcement learning method for a fairly particular setting.  The setting is one where the agent must solve some task for many episodes in a sequence, after which the task will change and the process repeats.  The proposed solution method splits the agent into two components, a master policy which is reset to random initial weights for each new task, and several sub-policies (motor primitives) that are selected between by the master policy every N steps and whose weights are not reset on task switches.  The core idea is that the master policy is given a relatively easy learning task of selecting between useful motor primitives and this can be efficiently learned from scratch on each new task, whereas learning the motor primitives occurs slowly over many different tasks.  To push this motivation into the learning process, the master policy is updated always but the sub-policies are only updated after an extended warmup period (called the joint-update or training period).  This experiments include both small domains (moving to 2D goals and four-rooms) and more complex physics simulations (4-legged ants and humanoids).  In both the simple and complex domains, the proposed method (MLSH) is able to robustly achieve good performance.
This approach to obtaining complex structured behavior appears impressive despite the amount of temporal structure that must be provided to the method (the choice of N, the warmup period, and the joint-update period).  Relying on the temporal structure for the hierarchy, and forcing the master policy to be relearned from scratch for each new task may be problematic in general, but this work shows that in some complex settings, a simple temporal decomposition may be sufficient to encourage the development of reusable motor primitives and to also enable quick learning of meta-policies over these motor-primitives.  Moreover, the results show that these temporal hierarchies are helpful in these domains, as the corresponding non-hierarchical methods failed on the more challenging tasks.
The paper could be improved in some places (e.g. unclear aliases of joint-update or training periods, describing how the parameters were chosen, and describing what kinds of sub-policies are learned in these domains when different parameter choices are made).