Remove .impl_UNBOXED() and functionalities associated with it (#49220)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49220
Since all ops are c10-full, we can remove .impl_UNBOXED now.
This also removes the ability of KernelFunction or CppFunction to store unboxedOnly kernels.
ghstack-source-id: 119450489
Test Plan: waitforsandcastle
Reviewed By: ezyang
Differential Revision: D25490225
fbshipit-source-id: 32de9d591e6a842fe18abc82541580647e9cfdad
