NeurIPS 2020

A Bayesian Nonparametrics View into Deep Representations

Review 1

Summary and Contributions: The paper attempts to fit a generative model on the latent representations obtained from CNNs and VAEs, as a method for further studying various important properties of these. To this end, the fit a Dirichlet process Gaussian mixture model (DP-GMM), and leverage its predictive density for the aforementioned purposes. This way, the paper yields very important insights, including that networks that can exploit patterns in data learn vastly less complex representations that networks forced to memorize, that standard VAE posteriors quickly collapses to the diagonal prior when regularization strength increases, while MMD-VAEs learn more complex posterior distributions, even with strong regularization, and that fitting a DP-GMM on latent representations yields an effective sampling strategy for latent codes, improving samples quality in VAEs with rich posteriors.

Strengths: The approach is very novel and interesting. The obtained insights are important. The methodological framework is valuable and opens new avenues for further research. The empirical results are plentiful and deep.

Weaknesses: I am quite happy with the paper as it stands.

Correctness: The used DP-GMM related formulas for training and inference are pretty standard. The derivations pertaining to the used metrics are correct.

Clarity: The paper is vert well written

Relation to Prior Work: The review of prior work is satisfactory.

Reproducibility: Yes

Additional Feedback: I have read the rebuttal; it hasn't changed my view on the paper.

Review 2

Summary and Contributions: [Update after author response: I want to thank the authors for their detailed response. In my opinion the authors have convincingly addressed many questions raised by R5, and have also addressed comments 1) and 5) raised by me - my comments 2) and 3) were suggestions for further improvements but were outside the scope of this very short rebuttal phase. I continue to vote and argue for accepting the paper and stand by my score. I also want to strongly encourage the authors to add/include some of the questions raised by R5 into the discussion of the camera-ready paper - I think that many of the questions were very sensible and readers from the broader ML community might have similar questions.] The paper proposes to model neural network activation (-statistics) with a Gaussian mixture model with an unknown number of mixture components and unknown parameters for each mixture component. Both of the latter are captured via a hierarchical model (Dirichlet Process prior, and Normal-Inverse-Wishart prior) and a computationally efficient scheme for fitting the model to data and computing the posterior predictive are introduced (using a mix of closed-form solutions and sampling). The model is used to investigate the complexity of neural network representations via the KL divergence between a max-entropy reference distribution and the model posterior. In a similar fashion the model is also used to measure complexity of beta-VAE latent-space activations and quantify their disentanglement by estimating the Total Correlation. Contributions: 1) The main findings (which each are interesting contributions) of the paper are: 1a) Significant differences in representation complexity between networks that were trained on standard data compared to networks trained on random labels. 1b) Qualitative differences between data-augmentation as a regularizer compared to dropout (often dropout representations have high complexity, whereas data-augmentation more consistently has low complexity). 1c) Investigating the beta parameter in a VAE reveals that model complexity as measured by the method behaves more or less as expected (strong regularization leads to very low complexity latent representations), whereas the effect is much less pronounced for a MMD-VAE. 1d) Finally, standard VAE representations are shown to become increasingly disentangled with increasing beta, which only holds true to a weak degree for the MMD-VAE. 2) The Nonparametric model introduced in the paper is solid and sophisticated. It builds to some degree on previous work, and is well described (given space limitations) and computationally tractable ways of computing quantities of interest are provided.

Strengths: To the best of my knowledge the model proposed in the paper along with the concrete inference scheme is novel. Relations to similar previously proposed models and schemes are nicely discussed. The model is interesting and novel, and the application in the paper of investigating representational complexity is timely and interesting. Experiments are conducted in a sound fashion and results look convincing. The paper contributes towards a bigger debate regarding the nature of solutions that are obtained by training neural networks via SGD - in particular how/why solutions differ vastly in complexity when training on random/unstructured data despite no other changes in regularization strength. This debate has continuously grown over the last years and has sparked numerous pieces of interesting theoretical and empirical work, which is highly relevant to large parts of the NeurIPS community.

Weaknesses: The paper does not state current limitations of the approach. Additionally it would be nice to perform some sanity-check experiments where the ground-truth complexity or posterior predictive distribution are known (to a large degree) and the proposed method can be gauged in terms of recovering this ground-truth. The experiments are limited to image classification and image generation, which naturally limits the generalizability of the findings somewhat. And while another non-vision application would be interesting, I think that the presented experiments are already quite extensive and conducted in a sound fashion (which I personally prefer over showing more different results in a less thorough fashion).

Correctness: To the best of my knowledge the claims and methodology shown in the paper are correct. However, I cannot fully rule out that I have overlooked something in the model - particularly since complex probabilistic models with approximate inference algorithms can easily have important subtleties that are hard to spot.

Clarity: The paper is mostly well written, though another pass for typos, etc. is needed. The description of the probabilistic model and the details of the inference scheme would perhaps benefit from a slightly expanded explanation, but I personally think that the author’s already did a very good job with their explanation given the limited space of the main paper. There is no part that I would currently recommend to push to the appendix to gain more space in the main paper.

Relation to Prior Work: Related work is concisely, but nicely discussed.

Reproducibility: Yes

Additional Feedback: Overall, I greatly enjoyed reading the paper. The main questions are timely, the approach and the proposed nonparametric model are novel and interesting and so are the main findings. I have some small suggestions and comments (see below). I currently vote and argue for accepting the paper, however I will of course take into account the other reviewer’s opinions and issues raised, as well as the author’s response. I am fairly confident in my verdict, but as pointed out earlier it might be that I missed a crucial subtlety or limitation of the probabilistic model and inference scheme, which would make me reconsider my verdict. Comments: 1) Please briefly state limitations of the proposed model, as well as limitations of the main findings due to the experimental setting shown in the paper. 2) Perhaps slightly expand the discussion of choosing a maximum-likelihood fit Gaussian as the base-distribution for computing computational complexity (the KL divergence to the posterior predictive). I am happy with the maximum entropy argument, but the particular choice might explain some of the results of the VAE experiment. In particular, the second term in the VAE objective is thought to play the role of a complexity regularizer - and of course, the MMD penalty implicitly uses a different notion of complexity than the (standard) KL divergence to a isotropic Gaussian. I personally would be very convinced if the main findings would also hold qualitatively under another reasonable choice of the base-distribution for measuring complexity (though as I said before I don’t consider this as necessary for the paper, just a suggestion). 3) It would be very interesting to investigate a VAE architecture with a very flexible posterior, such as a flow-based model, and contrast the complexity of its representations against VAEs with a more constrained parametric model of the posterior. (Again, this is a suggestion, but probably beyond the scope of the rebuttal phase). 4) It would be good to discuss relations between the proposed disentanglement score and other work discussing total correlation, e.g. Auto-Encoding Total Correlation Explanation, Gao et al. (I am not one of the authors) 5) Minor comment re. Line 225 - 227: I might be missing an important subtlety, but since all conditional posteriors are regularized with an input independent prior, it should be fairly straightforward that under sufficient regularization strength the conditional posteriors should become very similar to the prior, which is an isotropic Gaussian (and thus it should not be too surprising that the complexity measure in this case goes to zero). But I don’t think there’s a problem here - I’d rather see it as an interesting sanity-check for the method.

Review 3

Summary and Contributions: The authors propose a new complexity measure for latent representations in CNNs and VAEs. They show that certain networks (CNNs with random training data, CNNs with dropout, MMD-VAEs) learn more complex representations than others (standard CNNs, standard VAEs). Update: After reading the other reviews and the author response, I have understood some things that I was missing before. I have thus increased my score.

Strengths: - The question of how complex learned representations are is interesting and has not been studied in distribution across different initializations before.

Weaknesses: - Some of the modeling choices are not perfectly motivated and not compared against other options (see below). - The main weakness is that it remains unclear whether the representational complexity is good or bad and so why exactly we should care about it in this context (see below).

Correctness: The claims and method seem to be correct. However, some of the methodological choices remain not sufficiently motivated.

Clarity: The paper is well written.

Relation to Prior Work: The prior work is properly discussed.

Reproducibility: Yes

Additional Feedback: - It is a bit unclear why the DP-GMM is necessarily the best density estimator for this task. Why not a normal GMM with a fixed number of components? - Why is the KL with the Gaussian a good complexity measure? Why not the MMD for instance? Why not use a student-t instead of a Gaussian? - In the non-parametric DP-GMM, is the number of components itself not already a measure of complexity? How does it correlate with the proposed measure? - Why is the posterior predictive a mixture of student-t distributions? - What is the relation between the proposed measure in (10) and the mutual information between dimensions? Is it the same? Why not use the total correlation instead? - Is the complexity in CNN representations good or bad? Does it correlate with performance? For instance, CNNs trained on random labels should not generalize, so I would assume the complexity is bad there. But CNNs trained with dropout usually generalize better than standard CNNs and also have a higher complexity. - Is the lack of convergence to the prior in the MMD-VAE due to the choice of kernel? Would other kernels lead to a behavior more similar to the standard VAE? - Again, is the complexity in the VAE representation good or bad? Is the MMD-VAE supposed to be the better model? From the visual quality, it's hard to tell. What are the likelihoods? - Are the results robust w.r.t. changing the random labels of the training set? How much do the representations change when the labels change?