Restore FlexAttention and FlashV3 backward (#2473)
Summary: Pull Request resolved: https://github.com/pytorch/benchmark/pull/2473
Reviewed By: xuzhao9
Differential Revision: D63543625
Pulled By: bertmaher
fbshipit-source-id: 1693e15875544bda0f5f6c69daa5597fffd80509
