support empty batch in group normalization (#32401)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32401
https://github.com/pytorch/pytorch/issues/12013
Test Plan: buck test mode/dev-nosan //caffe2/test:nn -- 'test_GroupNorm_empty'
Differential Revision: D19463720
fbshipit-source-id: 8ae44590fc5eeb1adc69a2345d7cc2187d3307ac
