pyi cleanup (#49054)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49054
These are some followups from the first pyi codegen PR. Still maintaining byte-for-byte compatibility in this one.
- Separated `argument_str() with a pyi flag into two functions, `argument_str()` and `argument_str_pyi()`
- Added a notes section for pyi at the top of `python.py`
- Added a `Python Interface` section that I moved the free-standing pyi functions to
Test Plan: Imported from OSS
Reviewed By: ljk53
Differential Revision: D25410848
Pulled By: bdhirsh
fbshipit-source-id: db83a80af900c32b5e32d67ce27767f6e7c2adfb
