Move qkv_same, kv_same into branch (#30142)
Summary:
Perf improvements to multi_head_attention_forward
- qkv_same and kv_same were not used outside of that branch. Further, kv_same was calculated even though it is not used if qkv_same
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30142
Differential Revision: D18610938
Pulled By: cpuhrsch
fbshipit-source-id: 19b7456f20aef90032b0f42d7da8c8a2d5563ee3
