Fix method stub creation for function attributes (#37994)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37994
Before, reassigning a method in a module (like `forward = _forward`)
didn't work, because we look at the function object's name for our def
name when building AST. Mkae that overrideable to handle cases like
reassignment
Test Plan: Imported from OSS
Differential Revision: D21444535
Pulled By: suo
fbshipit-source-id: 4f045f18b5a146edc8005689af525d7d7ed8dd5f
