Disable check for dropout in MultiheadAttention fast_path (#88831)
Since we already enforce eval mode for the fast_path, we do not need to also check for a falsy dropout value, as a model trained with dropout will have a non-zero dropout during eval mode, even though it won't be applied.
Fixes #88806
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88831
Approved by: https://github.com/drisspg
