Paper:
We study the role of intrinsic motivation as an exploration bias for reinforcement learning in sparse-reward synergistic tasks, which are tasks where multiple agents must work together to achieve a goal they could not individually. Our key idea is that a good guiding principle for intrinsic motivation in synergistic tasks is to take actions which affect the world in ways that would not be achieved if the agents were acting on their own. Thus, we propose to incentivize agents to take (joint) actions whose effects cannot be predicted via a composition of the predicted effect for each individual agent. We study two instantiations of this idea, one based on the true states encountered, and another based on a dynamics model trained concurrently with the policy. While the former is simpler, the latter has the benefit of being analytically differentiable with respect to the action taken. We validate our approach in robotic bimanual manipulation and multi-agent locomotion tasks with sparse rewards; we find that our approach yields more efficient learning than both 1) training with only the sparse reward and 2) using the typical surprise-based formulation of intrinsic motivation, which does not bias toward synergistic behavior. Videos are available on the project webpage: https://sites.google.com/view/iclr2020-synergistic.

Review:
The paper proposes a novel algorithm for encouraging synergistic behavior in multi-agent setups with an intrinsic reward that promotes the agents to work together to achieve states that they cannot achieve individually without cooperation. The paper focuses on a two-agent environment where an approximate forward dynamics model is learnt for each agent, and can be composed sequentially to predict the next environment state given each agent’s action. However, this prediction will be inaccurate if the agent’s affected the environment state in such a way that individual dynamics model cannot predict i.e. synergistic behavior was produced. This prediction error is used as extrinsic reward by the proposed approach, while also having a variant where the true next state is replaced by another approximation of a joint forward model which allows for differentiability of actions with respect to the intrinsic reward. Empirical analysis shows that this intrinsic reward promotes synergetic behavior on two-agent robotic manipulation tasks and achieves better performance that baselines and ablations.
I vote for weak accept as the paper proposes a novel intrinsic reward for promoting synergetic behavior in multi-agent systems, while also demonstrating that such an intrinsic reward can be differentiable if a joint forward dynamics model is approximated in addition to individual forward dynamics models given each agent’s actions. The paper does not show experiments beyond 2 agents and the four robotic manipulation tasks have been shown to work when provided with generic skills as an action space, which requires hand-defining or learning by demonstration.
From the description of the random policy, it is stated that a random policy over skills serves as a sanity check to ensure that the skills do not trivialize the task. This seems to suggest that the extrinsic reward only baseline did not use these skills and was disadvantaged. Some clarification is required here - did the extrinsic reward only baseline use the same skills as the proposed method? If it did, it would obviate the need to have a random policy sanity check. 
The paper suggests a baseline for separate arm surprise. In a similar vein, why wasn’t a joint-arm surprise baseline employed, which can basically treat both arms as a single agent?
Synergistic behavior seems hard to achieve for a large number of agents, but the paper does not give insights into whether such an algorithm will work for more than 2 agents. Typically multi-agent systems in prior work have worked with a large number of agents in environments other than robotic manipulation - such experiments may help in strengthening the proposed method.