SumKernel (BFloat16): use float as accumulation type (#55217)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/55217
Test Plan: Imported from OSS
Reviewed By: ngimel
Differential Revision: D28836794
Pulled By: VitalyFedyunin
fbshipit-source-id: 46ed3a862c2bb4c6325c78ecfc5d01761f7a113a
