C++/Python API Parity: add AlphaDropout (#28424)
Summary:
- add `AlphaDropoutImpl` to `modules/dropout.h` and `modules/dropout.cpp`
- add `functional/dropout.h` containing the `alpha_dropout` function
- include `functional/dropout.h` in `nn/functional.h`
- add functional and module tests
- related issue https://github.com/pytorch/pytorch/issues/25883
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28424
Differential Revision: D18589162
Pulled By: yf225
fbshipit-source-id: c85734e02431a6c052515e26b11ca30ad7303644
