Preliminary ControlNet PR (WIP)

[rishab-partha]

Adds ControlNet to the diffusion repo for both SDXL and SD2 style models.

Some of the highlights of the work here:

1. Custom callback to handle initializing a ControlNet from a Composer UNet Checkpoint:
   Sample Workflow:
   a. We train a custom SDXL style model, with a checkpoint of UNet, VAE, text encoder.
   b. We want to initialize a ControlNet from this Composer checkpoint
   c. Challenge: FSDP wrapping happens before the checkpoint is loaded, meaning that we will get NaNs if we load directly
   d. Solution: Need to all gather the weights before restoring the checkpoint and duplicating weights

2. Class that handles both ControlNet style generation and forward (training passes) -- for both the forward and the generate, this involves generating the control image and feeding directly into the UNet.

3. Model classes for the two architectures that support loading directly from YAML files.

To-dos:
Show inference outputs here + add comments