Add stub for transformer.py and MultiheadAttention Class. (#28396)
Summary:
Add stub for `transformer.py` and `class MultiheadAttention`. Add import for `transformer.py` and `class MultiheadAttention` in `__init__.pyi.in`. I've tested the code hint in PyCharm and all works file.
Relate issue: [https://github.com/pytorch/pytorch/issues/27842](https://github.com/pytorch/pytorch/issues/27842)
ezyang
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28396
Differential Revision: D19300287
Pulled By: ezyang
fbshipit-source-id: 1a79d6518b5edd4643892c46a959108385c739ad
