add Int8SpatialBNRelu (#22014)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22014
Add Int8SpatialBN + Relu fused operator.
Reviewed By: dskhudia
Differential Revision: D15916551
fbshipit-source-id: a938e0f0e105ab5f823a3cb6144f50aa2ab944c1
