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Information-theoretic Generalization Analysis for VQ-VAEs: A Role of Latent Variables
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Derives decoder-independent, information-theoretic generalization bounds for VQ-VAE reconstruction loss using a supersample setting with a data-dependent prior over latent variables, plus a permutation-symmetric variant that converges asymptotically and a W2 bound on generated data.
Problem
Theoretical understanding of how latent variables and encoder complexity affect generalization of VQ-VAEs in unsupervised learning, where existing supersample IT bounds depend on the decoder and do not vanish with sample size.
Approach
Constructs a data-dependent prior over discrete latent indices via supersample shuffling of LVs, and extends to a permutation-symmetric setting over all 2n points, yielding bounds that depend only on encoder and latent-variable complexity (not the decoder) and that converge to zero with n.
Key results
- Permutation-symmetric bound removes the non-vanishing empirical KL term and yields gen(n,D)=O(sqrt(d_phi d_z log n / n)) via metric entropy, decoder-independent.
- On MNIST (K=128, d_z=64), the second KL term correlates positively with the generalization gap (r≈0.46–0.60), while the first empirical KL term/n does not decrease with n and correlates negatively.
- Adding a single ResBlock (~74,000 parameters) has a negligible effect on the generalization gap despite lower training loss for n≥1000.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Add one decoder ResBlock, introducing approximately 74,000 additional parameters | VQ-VAE on MNIST · MNIST | generalization gap | No clear effect |
| Increase the number of decoder ResBlocks | VQ-VAE · MNIST, n ≥ 1000 | training loss | Helped |
| Increase the number of decoder ResBlocks | VQ-VAE · MNIST | generalization gap | No clear effect |
| Use a learned prior approximating a data-dependent prior instead of the standard uniform prior | VQ-VAE · VQ-VAE data generation | test performance | Helped |
| Increase the sample size | VQ-VAE · MNIST | KL(Q_{J,U} || P) / n | No clear effect |
| Increase the sample size | VQ-VAE · MNIST | KL(Q_{J̃,U} || Q_{J̃}) / n | Helped |
Surprising findings
- The empirical KL term over latent variables does not vanish as n grows even under the optimal prior, and it is negatively correlated with the generalization gap.
- Increasing decoder complexity lowers training loss but leaves the generalization gap largely unchanged.
Limitations
- The permutation-symmetric bound is hard to compute numerically, so it is impractical as an evaluation metric.
- Analysis is limited to discrete latent spaces; continuous-latent extensions are non-trivial.
- Bounds assume the decoder can adequately fit the training data.
Experimental setup
Datasets: MNIST
Benchmarks: MNIST
Metrics: generalization gap KL divergence terms training loss test loss
Baselines: basic IT-bound (supersample) PAC-Bayes VAE bound fCMI-based bound
Builds on: supersample information-theoretic generalization bounds VQ-VAE Donsker-Varadhan inequality PAC-Bayes bounds for VAEs VampPrior
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