Paper:
Hierarchical Reinforcement Learning for Zero-shot Generalization with Subtask Dependencies
We introduce a new RL problem where the agent is required to generalize to a previously-unseen environment characterized by a subtask graph which describes a set of subtasks and their dependencies. Unlike existing hierarchical multitask RL approaches that explicitly describe what the agent should do at a high level, our problem only describes properties of subtasks and relationships among them, which requires the agent to perform complex reasoning to find the optimal subtask to execute. To solve this problem, we propose a neural subtask graph solver (NSGS) which encodes the subtask graph using a recursive neural network embedding. To overcome the difficulty of training, we propose a novel non-parametric gradientbased policy, graph reward propagation, to pre-train our NSGS agent and further finetune it through actor-critic method. The experimental results on two 2D visual domains show that our agent can perform complex reasoning to find a near-optimal way of executing the subtask graph and generalize well to the unseen subtask graphs. In addition, we compare our agent with a Monte-Carlo tree search (MCTS) method showing that our method is much more efficient than MCTS, and the performance of NSGS can be further improved by combining it with MCTS.

Review:
The paper introduces an RL problem where the agent is required to execute a given subtask graph which describes a set of subtasks and their dependency, and proposes a neural subtask graph solver (NSS) to solve this problem. In NSS, there are an observation module to capture the environment information using CNN, and a task module to encode the subtask graph using recursive-reverse-recursive neural network (R3NN). A non-parametric reward-propagation policy (RProp) is proposed to pre-train the NSS agent and further finetune it through actor-critic method.  Pros: 1. In general, the problem introduced in this paper is interesting and the method which uses CNN to capture the observation information and R3NN to encode the subtask graph is a good idea. 2. The method is evaluated in two experiments which outperforms the baselines in some different settings.  Cons: 1. Writing: many details of the proposed method are included in the supplementary material which makes it difficult to understand by reading the main paper only. For example, even the objective is not defined in the main paper and all the training processes are presented in the supplementary material. 2. The baseline used in this paper is simple. Why not compare to the schemes listed in the related work section?