Paper:
Exploration is a key component of successful reinforcement learning, but optimal approaches are computationally intractable, so researchers have focused on hand-designing mechanisms based on exploration bonuses and intrinsic reward, some inspired by curious behavior in natural systems. In this work, we propose a strategy for encoding curiosity algorithms as programs in a domain-specific language and searching, during a meta-learning phase, for algorithms that enable RL agents to perform well in new domains. Our rich language of programs, which can combine neural networks with other building blocks including nearest-neighbor modules and can choose its own loss functions, enables the expression of highly generalizable programs that perform well in domains as disparate as grid navigation with image input, acrobot, lunar lander, ant and hopper. To make this approach feasible, we develop several pruning techniques, including learning to predict a program's success based on its syntactic properties. We demonstrate the effectiveness of the approach empirically, finding curiosity strategies that are similar to those in published literature, as well as novel strategies that are competitive with them and generalize well.

Review:
This paper proposes to meta-learn a curiosity module via neural architecture search. The curiosity module, which outputs a meta-reward derived from the agent’s history of transitions, is optimized via black box search in order to optimize the agent’s lifetime reward over a (very) long horizon. The agent in contrast is trained to maximize the episodic meta-reward and acts greedily wrt. this intrinsic reward function. Optimization of the curiosity module takes the form of an epsilon-greedy search, guided by a nearest-neighbor regressor which learns to predict the performance of a given curiosity program based on hand-crafted program features. The program space itself composes standard building blocks such as neural networks, non-differentiable memory modules, nearest neighbor regresses, losses, etc. The method is evaluated by learning a curiosity module on the MiniGrid environment (with the true reward being linked to discovering new states in the environment) and evaluating it on Lunar Lander and Acrobot. A reward combination module (which combines intrinsic and extrinsic rewards) is further evaluated on continuous-control tasks (Ant, Hopper) after having been meta-trained on Lunar Lander. The resulting agents are shown to match the performance of some recent published work based on curiosity and outperforms simple baselines.
This is an interesting, clear and well written paper which covers an important area of research, namely how to find tractable solutions to the exploration-exploitation trade-off. In particular, I appreciated that the method was clearly positioned with respect to recent work on neural architecture search, meta-learning approaches to curiosity as well as forthcoming about the method’s limitations (outlining many hand-designed curiosity objectives which fall outside of their search space). There are also some interesting results in the appendix which show the efficacy of their predictive approach to program performance.
My main reservation is with respect to the empirical validation. Very few existing approaches to meta-learning curiosity scale to long temporal horizons and “extreme” transfer (where meta-training and validation environments are completely different). As such, there is very little in the way of baselines. The paper would greatly benefit from scaled down experiments, which would allow them to compare their architecture search approach to recent approaches [R1, R2], black-box optimization methods in the family of evolution strategies (ES, NES, CMA-ES), Thompson Sampling [R3] or even bandits tasks for which Bayes-optimal policies are tractable (Gittins indices). These may very well represent optimistic baselines but would help better interpret the pros and cons of using neural architecture search for meta-learning reward functions versus other existing methods. Conversely, the paper claims to “search over algorithms which [...] generalize more broadly and to consider the effect of exploration on up to 10^5, 10^6 timesteps” but at the same time does not attempt to show this was required in achieving the reported result. Pushing e.g. RL2 or Learning to RL baselines to their limits would help make this claim.
Along the same line, it is regrettable that the authors chose not to employ or adapt an off-the-shelf architecture search algorithm such as NAS [R4] or DARTS [R5]. I believe the main point of the paper is to validate the use of program search for meta-learning curiosity, and not the details of the proposed search procedure (which shares many components with recent architecture search / black-box optimization algorithms). Using a state-of-the-art architecture search algorithm would have made this point more readily.
Another important point I would like to see discussed in the rebuttal, is the potential for cherry-picking result. How were the “lunar lander” and “acrobot” environments (same question for “ant” and “hopper”) selected? From my understanding, it is cheap to evaluate learnt curiosity programs on downstream / validation tasks. A more comprehensive evaluation across environments from the OpenAI gym would help dispel this doubt. Another important note: top-16 results reported in Figure 4 and Table 1 are biased estimates of generalization performance (as they serve to pick the optimal pre-trained curiosity program). Could the authors provide some estimate of test performance, by e.g. evaluating the performance of the top-1 program (on say lunar lander) on a held-out test environment? Alternatively, could you comment on the degree of overlap between the top 16 programs for acrobot vs lunar lander? Thanks in advance.
[R1] Learning to reinforcement learn. JX Wang et al..
[R2] RL2: Fast Reinforcement Learning via Slow Reinforcement Learning. Yan Duan et al.
[R3] Efficient Bayes-Adaptive Reinforcement Learning using Sample-Based Search. Guez et al.
[R4] Neural Architecture Search with Reinforcement Learning. Barrett et al.
[R5] DARTS: Differentiable Architecture Search. Liu et al.