Paper:
Reinforced Continual Learning
Most artificial intelligence models are limited in their ability to solve new tasks faster, without forgetting previously acquired knowledge. The recently emerging paradigm of continual learning aims to solve this issue, in which the model learns various tasks in a sequential fashion. In this work, a novel approach for continual learning is proposed, which searches for the best neural architecture for each coming task via sophisticatedly designed reinforcement learning strategies. We name it as Reinforced Continual Learning. Our method not only has good performance on preventing catastrophic forgetting but also fits new tasks well. The experiments on sequential classification tasks for variants of MNIST and CIFAR-100 datasets demonstrate that the proposed approach outperforms existing continual learning alternatives for deep networks.

Review:
This paper presents a simple but effective way to dynamically grow a neural network in a continual learning setup. The framework uses reinforcement learning to learn a policy that outputs decisions in terms of number of nodes/filters to add, and those additional filters are trained specifically for the new task (while the previous layers are held constant to prevent reduction in performance for old tasks). This is progressively done layer by layer where previous decisions for earlier layers serve as input to an LSTM in order to make decisions for the subsequent layer. Results show similar performance to approaches such as DEM, but have some better results for CIFAR-100 especially and the method results in networks with lower complexity.   Strengths  - Good overview of two groups of approaches on this topic   - The idea of using RL for continual learning is a good one, and one that makes sense given recent advancements in RL for architecture search.   - The results seem positive, especially for CIFAR-100 and for lowering the overall complexity of the solutions.    - The method is simple, which means that it should be easy to implement, barring issues in RL training stability. Releasing source code would be very helpful in this regard for reproducing the results in the paper.  Weaknesses  - An argument against DEN, a competitor, is hyper-parameter sensitivity. First, this isn't really shown, but second (and more importantly) reinforcement learning is well-known to be extremely unstable and require a great deal of tuning. For example, even random seed changes are known to change the behavior of the same algorithm, and different implementation of the same algorithm can get very different results (this has been heavily discussed in the community; see keynote ICLR talk by Joelle Pineau as an example). This is not to say the proposed method doesn't have an advantage, but the argument that other methods require more tuning is not shown or consistent with known characteristics of RL.    * Related to this, I am not sure I understand experiments for Figure 3. The authors say they vary the hyper-parameters but then show results with respect to # of parameters. Is that # of parameters of the final models at each timestep? Isn't that just varying one hyperparameter? I am not sure how this shows that RCL is more stable.   - Newer approaches such as FearNet [1] should be compared to, as they demonstrated significant improvement in performance (although they did not compare to all of the methods compared to here).  [1] FearNet: Brain-Inspired Model for Incremental Learning, Ronald Kemker, Christopher Kanan, ICLR 2018.   - There is a deeper tie to meta-learning, which has several approaches as well. While these works don't target continual learning directly, they should be cited and the authors should try to distinguish those approaches. The work on RL for architecture search and/or as optimizers for learning (which are already cited) should be more heavily linked to this work, as it seems to directly follow as an application to continual learning.   - It seems to me that continuously adding capacity while not fine-tuning the underlying features (which training of task 1 will determine) is extremely limiting. If the task is too different and the underlying feature space in the early layers are not appropriate to new tasks, then the method will never be able to overcome the performance gap. Perhaps the authors can comment on this.  - Please review the language in the paper and fix typos/grammatical issues; a few examples:   * [1] "have limitation to solve" => "are limited in their ability to solve"   * [18] "In deep learning community" => "In THE deep learning community"   * [24] "incrementally matche" => "incrementally MATCH"   * [118] "we have already known" => "we already know"   * and so on  Some more specific comments/questions:   - This sentence is confusing [93-95] "After we have trained the model for task t, we memorize each newly added filter by the shape of every layer to prevent the caused semantic drift." I believe I understood it after re-reading it and the subsequent sentences but it is not immediately obvious what is meant.    - [218] Please use more objective terms than remarkable: "and remarkable accuracy improvement with same size of networks". Looking at the axes, which are rather squished, the improvement is definitely there but it would be difficult to characterize it as remarkable.    - The symbols in the graphs across the conditions/algorithms is sometimes hard to distinguish (e.g. + vs *). Please make the graphs more readable in that regard.  Overall, the idea of using reinforcement learning for continual learning is an interesting one, and one that makes sense considering recent advances in architecture search using RL. However, this paper could be strengthened by 1) Strengthening the analysis in terms of the claims made, especially with respect to not requiring as much hyper-parameter tuning, which requires more evidence given that RL often does require significant tuning, and 2) comparison to more recent methods and demonstration of more challenging continual learning setups where tasks can differ more widely. It would be good to have more in-depth analysis of the trade-offs between three approaches (regularization of large-capacity networks, growing networks, and meta-learning).   ==============================================  Update after rebuttal: Thank you for the rebuttal. However, there wasn't much new information in the rebuttal to change the overall conclusions. In terms of hyper-parameters, there are actually more hyper-parameters for reinforcement learning that you are not mentioning (gamma, learning rate, etc.) which your algorithm might still be sensitive to. You cannot consider only the hyper-parameter related to the continual learning part. Given this and the other limitations mentioned, overall this paper is marginally above acceptance so the score has been kept the same.