Revert "Flash Attention v2 (#105602)"
This reverts commit 9df3d882c8fe1e57914315aa250664ad5003d4fd.
Reverted https://github.com/pytorch/pytorch/pull/105602 on behalf of https://github.com/huydhn due to I think we miss a case here for sm80 build on inductor workflow as it is now OOM on trunk https://github.com/pytorch/pytorch/actions/runs/6042843139 ([comment](https://github.com/pytorch/pytorch/pull/105602#issuecomment-1701974862))
