Variational Autoencoders (VAEs) are one of the most commonly used generative models, particularly for image data. A prominent difficulty in training VAEs is data that is supported on a lower dimensional manifold. Recent work by Dai and Wipf (2019) suggests that on low-dimensional data, the generator will converge to a solution with 0 variance which is correctly supported on the ground truth manifold. In this paper, via a combination of theoretical and empirical results, we show that the story is more subtle. Precisely, we show that for linear encoders/decoders, the story is mostly true and VAE training does recover a generator with support equal to the ground truth manifold, but this is due to the implicit bias of gradient descent rather than merely the VAE loss itself. In the nonlinear case, we show that the VAE training frequently learns a higher-dimensional manifold which is a superset of the ground truth manifold.

本文研究了变分自编码器的训练问题，提出了一种二阶段的训练算法，证明了该算法可以在低维流形上训练，并且得到的生成器可以恰好支持原本的低维流形，且是由于训练算法的隐式偏差而非VAE损失本身的原因。

低维数据存在下的变分自编码器：优化空间及内在偏向