Move native MHA code out of PyTorch core (#72944)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/72944
Doesn't make sense to develop it in core right now.
ghstack-source-id: 149456040
Test Plan:
CI
run MHA benchmark in benchmark_transformers.py to make sure it doesn't crash
Reviewed By: zrphercule
Differential Revision: D34283104
fbshipit-source-id: 4f0c7a6bc066f938ceac891320d4cf4c3f8a9cd6
(cherry picked from commit b9df65e97cc357f2f7c3f3c114266fae33d2332a)
