[Caffe2] Fix spatial_bn bug for computing running_var on CPU or on CUDA without CuDNN (#42151)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42151
Previously our Caffe2 SpatialBN op impl was incorrect for computing running_var without unbias coefficent. Actually it should fail the test because the output will be different with CuDNN's output. However, our tests are too weak to find this bug. This diff fix all of them.
Test Plan: buck test mode/dev-nosan //caffe2/caffe2/python/operator_test:spatial_bn_op_test
Reviewed By: houseroad
Differential Revision: D22786127
fbshipit-source-id: db80becb67d60c44faae180c7e4257cb136a266d
