Adding flash attention for sequence parallel

[dianaml0]

Patch Description
Creating this PR off of #511, so it can be reviewed by @stephenroller

The last commit (3d709db) removes some changes from the sequence parallel code which enabled testing with world size of 1. CI is not currently running the test anyway because CI needs to be updated for the test to run.

The forward and backward tests are passing right now. However in some cases, about .2% of the elements fail

Testing steps
Unit Test gpu_tests/test_sequence_parallel_transformer_layer.py

[dianaml0]

CircleCI failure not related to this PR

[stephenroller]

Can we rebase for checks? Should we be concerned about the last bits of numerical differences?

[dianaml0]

@stephenroller just rebased the PR, should be up to date now. The rtol and atol used are the same ones we use for testing in xFormers for all flash attention bwds. I do a small training run to validate, would that be useful?

[dianaml0]

Looks like everything is passing now after rebasing

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