[pytorch] continue to rewrite gen_python_functions.py with typed models (#46978)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46978
Refactored and added type annotations to the most part of the file.
Some top-level codegen functions are called by other codegen scripts.
Will migrate them in subsequent PRs.
Test Plan: Imported from OSS
Reviewed By: ezyang
Differential Revision: D24589210
Pulled By: ljk53
fbshipit-source-id: e0c7e5b3672b41983f321400c2e2330d1462e76e
