基于KL和Renyi积分上界的变分自编码器学习（Learning in Variational Autoencoders with Kullback-Leibler and Renyi Integral Bounds）

This is an interesting workshop paper in ICML 2018 about variational autoencoders (VAE). I will introduce its main idea and some of my thinkings about this paper.

Background

Denote the input variable as $x$ and the latent variable as $z$. The decoder models the conditional probability distribution $p(x|z)$. Usually we assume the prior of $z$ is a normal Gaussian distribution. If the data distribution $p(x)$ is continuous, $p_\theta(x|z)$ is often defined as a parameterized Gaussian distribution $\mathcal{N}(x|\mu_x(z;\theta),\text{diag}[\sigma_x(z;\theta)])$. The goal of VAE is to optimize the log likelihood $$-\log p_\theta(x) = -\log \int p_\theta(x|z)p(z) dz.$$ However, because this objective is intractable, we instead optimize an upper bound of it $$\mathcal{L} = \mathbb{KL}[q_\phi(z|x)||p_\theta(z|x)] - \mathbb{E}[\log p_\theta(x|z)],$$ where $q_\phi(z|x)$ is any kind of variational distributions defined by the encoder. For more details about VAE, refer to my poster.

Drawback of VAE

One drawback of VAE is that sometimes the deviation given by the decoder $\sigma_x$ is very close to $0$. This is because $x$ usually lies in a low-dimensional manifold in the ambient space. The groundtruth probability density is $0$ off the manifold and infinite in the manifold. As