SyncBatchNorm Update on input dimension checks (#29626)
Summary:
update the requirements on input dimensions for `torch.nn.SyncBatchNorm`:
1. 2D inputs is now permissible, https://github.com/pytorch/pytorch/issues/20204 ;
2. requires at least two element along normalization plane (BatchNorm behavior);
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29626
Differential Revision: D18492531
Pulled By: albanD
fbshipit-source-id: f008e46a2d520d73c3c2730890a7424eba2ede9e
