Warn against callOp (#37797)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37797
This is slow (see comment in code).
Not fixing this yet, but at least adding a warning so people are aware and don't add new call sites.
ghstack-source-id: 103887226
Test Plan: waitforsandcastle
Differential Revision: D21390364
fbshipit-source-id: 7bff1c3b9756a16c9d9110f209c23bf557266dda
