flash_attention integration (#81434)
# Summary:
- I added a new submodule Cutlass pointing to 2.10 release. The inclusion of flash_attention code should be gated by the flag: USE_FLASH_ATTENTION. This is defaulted to off resulting in flash to not be build anywhere. This is done on purpose since we don't have A100 machines to compile and test on.
- Only looked at CMake did not attempt bazel or buck yet.
- I included the mha_fwd from flash_attention that has ben refactored to use cutlass 2.10. There is currently no backwards kernel on this branch. That would be a good follow up.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81434
Approved by: https://github.com/cpuhrsch
