add SpatialBN lowered fake fp16
Summary:
SpatialBNFakeLoweredFp16NNPI
this is the fake operator for SpatialBN that gets lowered into add/mul/div, etc.
Test Plan: test_spatialbn
Reviewed By: tracelogfb, amylittleyang
Differential Revision: D19658680
fbshipit-source-id: 2abddbcd9a2023ac75c494f20eaac2051b7139dc
