Add class-conditioning example notebooks.

fashionMNIST noise predictor

Merge branch 'master' of https://github.com/fastai/fastdiffusion

pure pytorch version from reading group, plus the sigma classifier from Jason

Initial commit

This seems to work fairly well...

More improvements

Merge branch 'master' of https://github.com/fastai/fastdiffusion

fashionmnist support and other changes

Fixing batch size assignment; Making model more memory efficient; Defaulting to mixed precision training.

More progress

Fixing encoder freeze/unfreeze function

Progress

More progress on this

Initial efforts on pretrained encoder based unet