Hide the OperatorKernel* argument from the stack based kernel API (#29337)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29337
This argument is needed by boxing wrappers so they're able to get a pointer to the corresponding unboxed kernel and call into it.
But if a kernel is registered in a boxed way, we don't need it and should hide this from the API.
This is especially needed for the backend fallback API where users would only be left wondering why this argument is there and what it does.
Also, hiding it allows us to potentially totally remove it in a future refactoring if we find some way to do so.
ghstack-source-id: 94481316
Test Plan: unit tests
Differential Revision: D18361991
fbshipit-source-id: 5cef26c896fe3f2a5db730d3bc79dcd62e7ef492
