Catchall kernels instead of fallback kernels (#20773)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20773
This removes the feature to register fallback kernels that are called when no other kernel matches.
Instead, we introduce the concept of catchall kernels that are always called independent of inputs.
If you only have a fallback/catchall kernel and no kernels with concrete dispatch keys, then both concepts behave in the same way.
The difference is that we now disallow operators to have both, a catchall kernel and kernels with concrete dispatch keys.
This was possible before when they have been fallback kernels.
The reason for this change is that we anticipate needing a method_missing feature in backends, i.e. a backend-wide fallback to call when the backend doesn't specify a kernel for an operator.
We are not clear on precendence between this backend-wide fallback and an operator level fallback. Disallow fallbacks for now so we are free to choose later without breaking backwards compatibility.
Reviewed By: dzhulgakov
Differential Revision: D15438977
fbshipit-source-id: cb3aa764a1659d909ee21a7bd8ec3d32438aafaa
