restore at::Half support for caffe2 SumOp (#41952)
Summary:
PR https://github.com/pytorch/pytorch/issues/40379 added long support but removed at::Half support. Restore at::Half support.
CC ezyang xw285cornell neha26shah
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41952
Reviewed By: colesbury
Differential Revision: D22720656
Pulled By: xw285cornell
fbshipit-source-id: be83ca7fe51fc43d81bc0685a3b658353d42f8ea
