C++ API TransformerEncoderLayer (#42633)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/42633
Test Plan: Imported from OSS
Reviewed By: ezyang
Differential Revision: D22994332
Pulled By: glaringlee
fbshipit-source-id: 873abdf887d135fb05bde560d695e2e8c992c946
