Revert D28836794: SumKernel (BFloat16): use float as accumulation type
Test Plan: revert-hammer
Differential Revision:
D28836794 (https://github.com/pytorch/pytorch/commit/4f5c68857f9f3dea107279cfc7cb9ff65fda84a6)
Original commit changeset: 46ed3a862c2b
fbshipit-source-id: 3b586eeb752b7cdee909fa97a4c78876a6014770
