[PyTorch] MHA: just use existing softmax on CPU (#72462)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/72462
Eliminating one potential source of bugs.
ghstack-source-id: 149067329
Test Plan: CI
Reviewed By: zrphercule
Differential Revision: D34006432
fbshipit-source-id: 55fda186636dc457db7f3f9c8e18f1627ff33b6a
(cherry picked from commit 5d8de9a12200db236d0fedfd3b13b1209fd4bc18)
