Throw error if multiple kernels registered (#20737)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20737
If someone tries to register multiple kernels in the same .op() call, we're now throwing an error.
Differential Revision: D15425660
fbshipit-source-id: 6d2f1444da3e16a6a98863d847965c2aa211e046
