Dependency refactor, add tests (#1165)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1165

Refactor dependency chain following:
https://docs.google.com/document/d/1vuU9UdQA5bq3WU7YnEgJowq9ey6vkF3KUKA0rpIxxIY/edit#heading=h.8raqyft9y50

**New dep chain:**
op_<op>.cpp -> selected_op_variants.h
selected_op_variants.h -> scalar_type_util

**Old dep chain:**
op_<op>.cpp -> scalar_type_util
scalar_type_util -> selected_op_variants.h

**Testing**
Add e2e test with addmul model
Update unit test to use addmul model

Gated to only run with fbcode, as there is dependency on exir which is only in fbcode.

Reviewed By: larryliu0820

Differential Revision: D51036567

fbshipit-source-id: ba76b3c61fc9c044232b0ae66f4dba788254bfab