NeurIPS 2020

Firefly Neural Architecture Descent: a General Approach for Growing Neural Networks

Review 1

Summary and Contributions: This paper propose an iterative optimization solution to grow neural networks in the architecture neighborhood to jointly optimize the network’s parameters and architecture. The proposed framework achieves similar or better performance compared to related works. The main contribution of this paper is defining an architecture search neighborhood and extending the searching by splitting existing neurons to jointly optimize splitting existing neurons, growing new neurons and growing new layers, and optimize the network parameter sets and architecture iteratively.

Strengths: +This paper introduces a method to apply “architecture descent” to iteratively search the network parameter set and architecture neighborhood to find the next optimal architecture; the theoretical grounding is sound. +This paper extends the network growing method from splitting existing neurons to include search of growing new neurons and growing new layers. It proposes a novel approach to find a functional neighborhood. +The baseline comparison experiments show the performance comparable to state of arts with similar accuracy and better search time.

Weaknesses: -This paper did not compare directly to the most related growing networks [1]. Instead it compared with 2 years ago's work (DARTS). -This paper only validate experiments on CIFAR-10 and CIFAR-100 which cannot explain the performance of the found architecture. - If the paper didn't validate on Imagenet, at least it can be validated on NAS-Bench-101, 201 and others. [2,3]. But this paper didn't provide a comprehensive study of these comparison. - Therefore, this experiments can't support the claim of "learns networks that are smaller in size but have higher average accuracy than those learned by the state-of-the-art methods." [1] AutoGrow: Automatic Layer Growing in Deep Convolutional Networks, KDD2020 [2] NAS EVALUATION IS FRUSTRATINGLY HARD, ICLR2020 [3]Ying, Chris, et al. "Nas-bench-101: Towards reproducible neural architecture search." International Conference on Machine Learning. 2019.

Correctness: Yes

Clarity: Yes

Relation to Prior Work: Yes

Reproducibility: Yes

Additional Feedback:

Review 2

Summary and Contributions: ################################################ I have read the rebuttal and other reviews. I think the authors adequately addressed these points in the response, and I would lean towards acceptance at this point. ################################################ The paper proposes an algorithm for growing neural networks while jointly optimizing the network parameters and architecture. The architectures can be grown more flexibly and to be more resource-efficient than prior work.

Strengths: (1) The paper is well written. (2) Optimizing neural network architectures is an important problem (3) Dynamically growing neural networks online is an important problem in optimizing network architecture. (4) The algorithm seemed to provide a reasonable performance boost for continual learning.

Weaknesses: (1) It’s unclear to me if the results in “Cell-Based Neural Architecture Search” are significant. ENAS/DARTS frequently perform worse than random architectures from their search space - [1]. But, these are also fairly different architecture search algorithms. Thus, I believe the key number is the difference between the random search. However, I would want more details about the random search is performed to see if this is reasonable. (2) How are the hyperparameters for your algorithm selected? (3) It would be nice to have an explicit related work section. [1] Adam, George, and Jonathan Lorraine. "Understanding neural architecture search techniques." arXiv preprint arXiv:1904.00438 (2019).

Correctness: The claims and methods seem correct to me.

Clarity: The paper is well written and clear for the most part - I think this is a strength of the paper.

Relation to Prior Work: It is difficult to assess how the work differs from previous contributions. The differences are mixed throughout the paper, but I would have appreciated a related work section.

Reproducibility: Yes

Additional Feedback: These comments did not affect my score but may help improve the paper. 29: growing -> growth 213: splittting -> splitting

Review 3

Summary and Contributions: The paper proposes a growing algorithm (Firefly) to increase the width and depth of a neural network. The Firefly increases the width/depth by using the idea of Splitting Steepest Descent [10] and adding brand new neurons/layers. The effectiveness of the method is demonstrated in a broad scope of applications such as Neural Architecture Search, Continual Learning, and searching efficient and accurate models, comparing with state-of-the-art work. The two key factors of the success are: (1) a gradient-based optimization, which optimally identifies which new neurons/layers can minimize the loss more; (2) newly added neurons/layers drop in gradually near the local minimum of the smaller net without dramatically change the loss of the small net.

Strengths: 1. a well explainable growing algorithm with efficient growing speed; 2. experiments on many applications comparing with state-of-the-art work.

Weaknesses: The writing quality and missing details degrade my rating of the paper: 1. The mismatch between math equations and intentions: 1.1. In Line 90, I bet f_t (x) should be the final output (a vector/scalar) of a net, but the sum is just an input to a neuron in the next layer; 1.2. The use of \epsilon and \varepsilon is ambiguous. I bet \epsilon is a small threshold such that new neurons don't change the loss much. However, in Line 101 & 105, I bet \varepsilon should be used. 1.3. in Step Two between Line 121 and Line 122, for a standard Taylor approximation, I bet s_i should be just a \Delta L. Please explain why it is an integration. 1.4. between Line 166 and Line 167, readers can be confused if f_{1:t} (x) is a sequence of functions from step 1 to step t, or just a function at step t. If the latter, what's its difference from f_t (x)? 1.5. "the candidate set of f_t+1 should consist of" is confusing. Why a set is just a function? 2. some important but missing details (see comments in the "Clarity")

Correctness: No crucial errors.

Clarity: 1. clarify if \varepsilon and \delta are learnable parameters the same as model parameters or they are just learnable during the architecture descent. In Line 116, when optimizing \varepsilon and \delta, are neural network weights also updated? 2. "measured by the gradient magnitude", magnitude of full-batch or a few mini-batches? 3. clarify if L is training loss or validation loss. 4. clarify z in Line 124. A small number? 5. Make use the legend labels "Random (split)" and "RandSearFh" in Fig. 3(a) are exactly the same with those appeared in the text ("RandSearch (split)" and "RandSearch (split+new)"). In Fig. 3(a), a should-have simple baseline: add one neuron and randomly initialize new weights. 6. In Figure 3(b), If the splitting and growing happen at the same time, the number of neurons (markers along x-axis) should have a gap larger than 1. Why did the markers appear at all x locations? 7. In Line 207, clarify the depth of the net. 8. In Figure 5/Line 249, cite and clarify baselines of EWC, DEN and RCL, where are not clarified/mentioned anywhere. Moreover, why other baselines are not curves but single dots. 9. The x-axis with 20 tasks doesn't match the caption "on 10-way split"

Relation to Prior Work: Introduction includes some related works. Related Work can be added if there is space and you might. Some content between Line 166 and 170 is basically a repeat. Trimming this can release some space. Consider the following related work: Dai, Xiaoliang, Hongxu Yin, and Niraj K. Jha. "NeST: A neural network synthesis tool based on a grow-and-prune paradigm." IEEE Transactions on Computers 68, no. 10 (2019): 1487-1497. Philipp, George, and Jaime G. Carbonell. "Nonparametric neural networks." arXiv preprint arXiv:1712.05440 (2017).

Reproducibility: No

Additional Feedback: 1. Line 97, \theda should be \theda_i 2. “A neurons is determined” -> "A neuron is determined" ================== Thanks for the clarification. Recap: this is an interesting paper with a broad of experimental evaluations. If the AC trusts the authors will improve the writing quality with (lots of) efforts, my final rate will go to 7.

Review 4

Summary and Contributions: This paper proposed a simple but highly flexible framework for progressively growing neural networks in a principled steepest descent fashion. The paper also demonstrates the effectiveness of our method on both growing the network on a single task and continual learning problems.

Strengths: The paper is easy to understand and interesting. The empirical evaluation is also quite convincing. Neural network architecture search is an important topic.

Weaknesses: There is no discussion of time complexity or space complexity of the proposed method. The method seems quite complicated and difficult to implement. And it's also surprising that the method can achieve better performance with shorter time usage.

Correctness: Yes

Clarity: Yes

Relation to Prior Work: The author discussed previous works and also have strong baselines.

Reproducibility: Yes

Additional Feedback: