Remove hacky_wrapper from BackendSelect kernels (#44062)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44062
Previously, BackendSelect kernels were still written in the legacy way, i.e. they took one TensorOptions argument instead of scattered dtype, layout, device, pin_memory, and they used hacky_wrapper to be callable. This caused a re-wrapping step. Calling into a BackencSelect kernel required taking the individual scattered arguments, packing them into a TensorOptions, and the kernel itself then gathered them again for redispatch.
Now with this PR, BackendSelect kernels are written in the new way and no hacky_wrapper or rewrapping is needed for them.
ghstack-source-id: 112825789
Test Plan:
vs master: https://www.internalfb.com/intern/fblearner/details/216117032/
vs previous diff: https://www.internalfb.com/intern/fblearner/details/216170194/
Reviewed By: ezyang
Differential Revision: D23484192
fbshipit-source-id: e8fb49c4692404b6b775d18548b990c4cdddbada
