Add code for SPADE VAE-GAN

[virginiafdez]

Code the SPADE VAE-GAN network in a MONAI friendly way.
Reference: https://arxiv.org/abs/1903.07291
SPADE is a [VAE]-GAN using a multi-scale discriminator for training. Coding for the VAE-GAN, using SPADE norm (to be implemented in issue #372) is required.