Implement `CustomDiffusionAttnProcessor2_0`. (#4604)
* Implement `CustomDiffusionAttnProcessor2_0`
* Doc-strings and type annotations for `CustomDiffusionAttnProcessor2_0`. (#1)
* Update attnprocessor.md
* Update attention_processor.py
* Interops for `CustomDiffusionAttnProcessor2_0`.
* Formatted `attention_processor.py`.
* Formatted doc-string in `attention_processor.py`
* Conditional CustomDiffusion2_0 for training example.
* Remove unnecessary reference impl in comments.
* Fix `save_attn_procs`.
