Update flash_attention kernel from 2.3.6 to 2.5.5 (#118935)
# Summary
Updates FlashAttention kernel code from tag [2.3.6](https://github.com/Dao-AILab/flash-attention/releases/tag/v2.3.6) to [2.5.3](https://github.com/Dao-AILab/flash-attention/releases/tag/v2.5.5).
The usual changes were then re-rellod on top of the modified kernel, changing how dropout saved for backward, removing the head_dim_pad since this would make the kernel inplace mutate and that has a bad interaction with functionalization.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/118935
Approved by: https://github.com/cpuhrsch
