prepare to split transformer header file (#43069)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43069
The transformer c++ impl need to put TransformerEncoderLayer/DecoderLayer and TransformerEncoder/TransformerDecoder in different header since TransformerEncoder/Decoder's options class need TransformerEncoderLayer/DecoderLayer as input parameter. Split header files to avoid cycle includsion.
Test Plan: Imported from OSS
Reviewed By: yf225
Differential Revision: D23139437
Pulled By: glaringlee
fbshipit-source-id: 3c752ed7702ba18a9742e4d47d049e62d2813de0
