[wip] Adding flash attention for sequence parallel

[stephenroller]

Patch Description
Since we're doing manual activation checkpointing, we need to have custom backwards for MHA. This patch leverages the flash implementation in xformers.

TODO:

• Gate behind the appropriate (existing) flag, and allow vanilla implementation to still exist
• Add a test

Testing steps
At very large scale, was a ~0.5-1% speedup. Probably not worth it at the largest scales given the risk of numeric changes, but maybe still worth it for medium scales.

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