Cannot use SyncBN with sharded DDP.
🐛 Bug
To Reproduce
Using SyncBN with sharded DDP plugin makes this error message.
AttributeError: SyncBatchNorm is only supported within torch.nn.parallel.DistributedDataParallel

I think this is not a PL bug but a bug of PyTorch and fair-scale.
Nevertheless, I think there is a way to support this combination in PL (like overriding SyncBN).
Do you have any update plans?
Environment
 - PyTorch Version (e.g., 1.0): 1.7.1
 - OS (e.g., Linux): Ubuntu 20.04
 - How you installed PyTorch (`conda`, `pip`, source): pip
 - Python version: 3.8
 - CUDA/cuDNN version: 10.1
 - GPU models and configuration: V100 / driver == 418.39
 - Any other relevant information: PL 1.1.0