Added .pyi file for flatten (#24459)
Summary:
Generated with `stubgen` and moved from `out/flatten.pyi` to `flatten.pyi.in`.
https://github.com/pytorch/pytorch/pull/22245#issuecomment-521875658
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24459
Differential Revision: D16881182
Pulled By: ezyang
fbshipit-source-id: 5e25fad55f169b5a58ab7522b583d7c923314d4d
