Revert "Update flash_attention kernel from 2.3.6 to 2.5.5 (#118935)"
This reverts commit 4b7a521856ca5fb0fc28edd18591f77fff5a6ba1.
Reverted https://github.com/pytorch/pytorch/pull/118935 on behalf of https://github.com/atalman due to Significantly increases build time. Optimization is needed ([comment](https://github.com/pytorch/pytorch/pull/118935#issuecomment-1971723284))
