fix sph batchnorm to use sph fma
Summary: make use of springhill's fma on SpatialBatchnorm
Test Plan:
re-enabled the unit test, ran it a couple of times
pending: net runner
Reviewed By: amylittleyang
Differential Revision: D20227767
fbshipit-source-id: 7c601f185940249c0a32bdf95d74a20552cd2625
