C++ APIs TransformerEncoder (#43187)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/43187
Test Plan: Imported from OSS
Reviewed By: zou3519
Differential Revision: D23182770
Pulled By: glaringlee
fbshipit-source-id: 968846138d4b1c391a74277216111dba8b72d683
