(#14580)

(#14580)

Summary:
Removes cast of half to float in torch.sum, with float16 input tensor and
float32 output tensor, instead we cast data when loading input in kernel.

This supposingly would save a kernel launch as well as a full global memory load
on promoted data type (float).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14580

Differential Revision: D13356203

Pulled By: ezyang

fbshipit-source-id: 85e91225b880a65fe3ceb493371b9b36407fdf48