simpler 'cpu_scatter_gather_base_kernel' (#34690)
Summary:
Simplifies `cpu_scatter_gather_base_kernel` to accept only binary operations and spares them from doing redundant checks.
CC v0dro
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34690
Differential Revision: D20604814
Pulled By: ngimel
fbshipit-source-id: 5e22c2f39a8e2861dc763454c88796d1aa38d2eb
