BackendSelect: Use at::_ops API and per-operator headers (#69840)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/69840
Test Plan: Imported from OSS
Reviewed By: jbschlosser
Differential Revision: D33160027
Pulled By: albanD
fbshipit-source-id: 0e492ec8bab73da90afd9df70f48c17a8206a768
(cherry picked from commit 133ec77e9f970fa042ce6fd3fd85c888334f8086)
