[fx] add type annotations (#43083)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43083
This adds type annotations to all classes, arguments, and returns
for fx. This should make it easier to understand the code, and
encourage users of the library to also write typed code.
Test Plan: Imported from OSS
Reviewed By: ezyang
Differential Revision: D23145853
Pulled By: zdevito
fbshipit-source-id: 648d91df3f9620578c1c51408003cd5152e34514
