[Feature] Support xformers

[okotaku]

Motivation

Support xformers. https://github.com/facebookresearch/xformers

Results (Optional)

Environment:

• A6000 Single GPU
• nvcr.io/nvidia/pytorch:23.10-py3

Settings:

• 1epoch training.

Model	total time
stable_diffusion_xl_pokemon_blip (fp16)	12 m 37 s
stable_diffusion_xl_pokemon_blip_xformers	10 m 6 s
stable_diffusion_xl_pokemon_blip_fast	9 m 47 s

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

---

📚 Documentation preview 📚: https://DiffEngine--105.org.readthedocs.build/en/105/