[pytorch] Make mobile find_method return an optional (#43965)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43965
As part of a larger effort to unify the API between the lite interpreter and full JIT:
- implement torch::jit::mobile::Method, a proxy for torch::jit::mobile::Function
- add support for overloaded operator() to mobile Method and Function
- mobile find_method now returns a c10::optional<Method> (so signature matches full jit)
- moves some implementation of Function from module.cpp to function.cpp
ghstack-source-id: 111161942
Test Plan: CI
Reviewed By: iseeyuan
Differential Revision: D23330762
fbshipit-source-id: bf0ba0d711d9566c92af31772057ecd35983ee6d
