Paper:
Conventionally, model-based reinforcement learning (MBRL) aims to learn a global model for the dynamics of the environment. A good model can potentially enable planning algorithms to generate a large variety of behaviors and solve diverse tasks. However, learning an accurate model for complex dynamical systems is difficult, and even then, the model might not generalize well outside the distribution of states on which it was trained. In this work, we combine model-based learning with model-free learning of primitives that make modelbased planning easy. To that end, we aim to answer the question: how can we discover skills whose outcomes are easy to predict? We propose an unsupervised learning algorithm, Dynamics-Aware Discovery of Skills (DADS), which simultaneously discovers predictable behaviors and learns their dynamics. Our method can leverage continuous skill spaces, theoretically, allowing us to learn infinitely many behaviors even for high-dimensional state-spaces. We demonstrate that zero-shot planning in the learned latent space significantly outperforms standard MBRL and model-free goal-conditioned RL, can handle sparse-reward tasks, and substantially improves over prior hierarchical RL methods for unsupervised skill discovery. Video demonstration of our results are available at: https://sites.google.com/view/dads-skill Figure 1: A humanoid agent discovers diverse locomotion primitives without any reward using DADS. We show zero-shot generalization to downstream tasks by composing the learned primitives using model predictive control, enabling the agent to follow an online sequence of goals (green markers) without any additional training.

Review:
This paper proposes a novel approach to learn a continuous set of skills (where a skill is associated with a latent vector and the skill policy network takes that vector as an extra input) by pure unsupervised exploration using as intrinsic reward a proxy for the mutual information  between next states and the skill (given the previous state). These skills can be used in a model-based planning (model-predictive control) with zero 'supervised' training data (for which the rewards are given), but using calls to the reward function to evaluate candidate sequences of skills and actions. The proposed approach is convincingly compared in several ways to both previous model-based approaches and model-free approaches.
This is a very interesting approach, and although I can already think of ways to improve, it seems like an exciting step in the right direction to develop more autonomous and sample efficient learning systems. I suggest to rate this submission as 'accept'.
Regarding the comparison to model-free RL: although it is true that no task-specific training is needed, a possibly large number of calls to the reward functions are needed during planning. It would be good to compare those numbers with the number of rewarded trajectories needed for the model-free approaches.
My main concern with the proposed method is how it would scale when the state-space becomes substantially larger (than the 2 dimensions x and y used in the experiments). The reason I am concerned is that the proposed method uses brutal sampling to search for good trajectories in z-space and action-space. It looks like the curse of dimensionality will quickly make this approach unfeasible. Also, it would be nice to have the learning system discover the important dimensions in which to plan (the x and y in the experiments), rather than having to provide them by hand.
A minor concern is the following: is it possible that the optimization could end up discovering a large number of highly predictable (and diverse) but useless skills?
In the related work section, 1st paragraph, in the list of citations, it might be good to also include the work on maximizing mutual information between representation of the next state and representation of the skill (Thomas et al, arXiv:1802.09484).
The definition of Delta (page 9) is strange: it is said that Delta should be minimized but Delta is defined as proportional to the rewards (which should be maximized). Maybe a sign is missing. Also, why not simply define the rewards as being normalized in the first place, so that the metric IS the accumulated reward rather than this unusual normalized version of it.