Revert "Revert "Flash Attention v2 (#105602)" (#108827)"
This reverts commit 24e9bbe22af296048f8242c6112d13cff726c588.
Reverted https://github.com/pytorch/pytorch/pull/108827 on behalf of https://github.com/huydhn due to I need to land this revert properly as there are new failures showing up on trunk ([comment](https://github.com/pytorch/pytorch/pull/108827#issuecomment-1711020924))
