[fx] enabling typechecking of fx files (#43082)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43082
Fixes all present errors in mypy. Does not try to add annotations everywhere.
Test Plan: Imported from OSS
Reviewed By: jamesr66a
Differential Revision: D23145854
Pulled By: zdevito
fbshipit-source-id: 18e483ed605e89ed8125971e84da1a83128765b7
