Re enabling xformers

[barbayrak]

I was using fast-stable-diffusion notebook of yours and I realized that your train_dreambooth.py was not using xformers so I checked out your code and the original diffusers. Based on this commit (https://github.com/huggingface/diffusers/pull/1627) I also added xformers support. After adding this, my training speed is increased from 1.35s/it to 1.04s/it

[TheLastBen]

I'm getting 1.01it/s with the current setup at 512x512