Check SM version before calling flash attention with BFloat16 (#86600)
The flash attention code path requires sm80 or newer to run on
BFloat16, so any OpInfo tests running with BFloat16 would fail with
the error:
```
RuntimeError: Expected q_dtype == at::kHalf || (is_sm8x && q_dtype == at::kBFloat16) to be true, but got false.
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86600
Approved by: https://github.com/ngimel
