[Caffe2] Fix LayerNormOp when batch_size == 0. (#45250)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45250
[Caffe2] Fix LayerNormOp when batch_size == 0.
Test Plan: buck test mode/dev-nosan //caffe2/caffe2/python/operator_test:layer_norm_op_test
Reviewed By: houseroad
Differential Revision: D23892091
fbshipit-source-id: 9a34654dd6880c9d14b7111fcf850e4f48ffdf91
