Fallback kernels (#18443)

Fallback kernels (#18443)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18443

Allow registering a kernel without a dispatch key. In this case, the kernel becomes a fallback kernel that is called whenever no other kernel matches.
This is also useful for the legacy function based API (since that API doesn't know about dispatch keys) or any other custom ops that don't care about dispatch
and just want one kernel to be called no matter the dispatch key.

Reviewed By: dzhulgakov

Differential Revision: D14603258

fbshipit-source-id: 242dc8871dad2989ca25079854d0cc97429e7199