Delete ATenDispatch (#28468)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28468
We don't need this anymore.
ghstack-source-id: 92595388
Test Plan: unit tests
Differential Revision: D18073339
fbshipit-source-id: d0ef1332c83e47117fe0a5eadc8faedb259cfba0
