Replace Slot on script::Method with NamedIValue (#18252)
Summary:
This refactor lets us track the types of initial values added onto a `Method`. The main motivation for this is the change in `module.cpp`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18252
Differential Revision: D14673459
Pulled By: driazati
fbshipit-source-id: 21200180c47f25bb70898771adfb569856e6c34a
