Add type stubs to import 'nn' modules (#22411)
Summary:
Forgot to mirror the `nn/ __init__.py` semantics in the new `nn` type stub.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22411
Differential Revision: D16149798
Pulled By: ezyang
fbshipit-source-id: 0ffa256fbdc5e5383a7b9c9c3ae61acd11de1dba
