[Caffe2] Remove explicitly divide by zero in SpatialBN training mode (#42380)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42380
[Caffe2] Remove explicitly divide by zero in SpatialBN training mode
Test Plan: buck test mode/dev-nosan //caffe2/caffe2/python/operator_test:spatial_bn_op_test
Reviewed By: houseroad
Differential Revision: D22873214
fbshipit-source-id: 70b505391b5db02b45fc46ecd7feb303e50c6280
