Type-annotate serialization.py (#40862)
Summary:
Move Storage class from __init__.pyi.in to types.py and make it a protocol, since this is not a real class
Expose `PyTorchFileReader` and `PyTorchFileWriter` native classes
Ignore function attributes, as there are yet no good way to type annotate those, see https://github.com/python/mypy/issues/2087
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40862
Differential Revision: D22344743
Pulled By: malfet
fbshipit-source-id: 95cdb6f980ee79383960f306223e170c63df3232
