Move unboxed dispatch decision into dispatcher (#29200)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29200
Before, the dispatch key for unboxed operators from native_functions.yaml was generated in codegen and passed to the c10 dispatcher.
Now, we generate it inside of the dispatcher, right next to where the same thing happens for boxed calls.
ghstack-source-id: 93371022
Test Plan: unit tests
Differential Revision: D18282747
fbshipit-source-id: 96a97fe83778d0a9e61b4441d6e2aed10d73209c
