Add losses for VAE

class VAE_KLDivergence(Loss):

  """The KL_divergence between hidden distribution and normal distribution.

  This loss implements KL divergence losses with normal distribution 

  This loss represents KL divergence losses between normal distribution(using parameter of distribution)

  based on "Auto-Encoding Variational Bayes" (https://arxiv.org/abs/1312.6114).

  The logvar should have shape (batch_size, hidden_space) and each term represents

class ShannonEntropy(Loss):

  """The ShannonEntropy of discrete-distribution.

  This loss implements shannon entropy based on

  This loss represents shannon entropy based on

  "A Brief Introduction to Shannon's Information Theory" (https://arxiv.org/abs/1612.09316).

  The inputs should have shape (batch size, num of variable) and represents