[PyTorch] Add native fast path for transformer encoder inference (#76333)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76333
The current PyTorch multi-head attention and transformer
implementations are slow. This should speed them up for inference.
ghstack-source-id: 154737857
(Note: this ignores all push blocking failures!)
Test Plan: CI
Reviewed By: cpuhrsch
Differential Revision: D35239925
fbshipit-source-id: 5a7eb8ff79bc6afb4b7d45075ddb2a24a6e2df28
