Fix warning in register_c10_ops by making index unsigned (#20964)
Summary:
Just an annoying warning that's been popping up a lot.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20964
Differential Revision: D15531064
Pulled By: Chillee
fbshipit-source-id: 9580115676c5e246481054bbfc749a551a3cca5e
