Introduce causal mask (#90508)
Summary: Introduce causal mask
This PR introduces a causal mask option _causal_mask (as well as causal mask detection if attn_mask is provided), since current custom kernels do not support arbitrary masks.
Test Plan: sandcastle & github ci/cd
Differential Revision: D41723137
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90508
Approved by: https://github.com/albanD
