Autoencoder implementation and training

[coryMosaicML]

This PR adds an implementation of the latent diffusion autoencoder, loss function, and training setup for training the autoencoder in composer.

• Basic network blocks are in diffusion/models/layers.py
• Complete networks, loss, and the and composer models are in diffusion/models/autoencoder.py
• For convenience in visualizing image reconstructions, there's a new image logging callback in diffusion/callbacks/log_diffusion_images.py
• diffusion/algorithms/discriminator_schedule.py contains a composer algorithm for switching on the discriminator.
• diffusion/train.py also contains some modifications to configure the param groups for training the autoencoder.

[Skylion007]

Some nits

[Landanjs]

Looks good overall! The only bug I noticed was in the n layer discriminator and unlikely for us to hit it.

Most of my other comments are about where specific parameter settings came from. I'm sure for most of these you already thought about them and made a decision, so feel free to ignore my comments if they aren't important