[nnc] Move operator implementations into a subdirectory (#59988)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59988
As we broaden operator support, putting all the implementations into
kernel.cpp is getting unwieldy. Let's factor them out into the "operators"
subdirectory.
This diff is big but it's entirely code movement; I didn't change anything,
other than to expose a few utilities in kernel.h.
ghstack-source-id: 131405139
Test Plan: CI
Reviewed By: ZolotukhinM
Differential Revision: D29115916
fbshipit-source-id: ba0df1d8dd4a108b584da3baf168407e966b2c78
