Convert mul to use opmath_gpu_kernel_with_scalars (#64019)

Convert mul to use opmath_gpu_kernel_with_scalars (#64019)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64019

Note that previously the functor operated on scalar_t and
this modifies it to operate on opmath_t, but this is not
a problem as half precision was implemented by performing the
compute in float anyway.

Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Test Plan: Imported from OSS

Reviewed By: ngimel

Differential Revision: D30575282

Pulled By: ezyang

fbshipit-source-id: cc6900ef996e755740afe48f9cb4d0366858dd47