Delete ATen/stub (#31763)
Summary:
This folder contained an empty CombinedStub file which isn't explicitly used anywhere.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31763
Differential Revision: D19262563
Pulled By: ezyang
fbshipit-source-id: 5d095c93d6f7a1cc35f5919aa6006b31c2376b18
