Add warning about not supporting torch.nn.SyncBatchNorm (aws#5046)

* Add warning about not supporting

* update wording

---------

Co-authored-by: pintaoz <[email protected]>

Author: pintaoz-aws

doc/frameworks/pytorch/using_pytorch.rst

@@ -375,6 +375,9 @@ To initialize distributed training in your script, call
 `torch.distributed.init_process_group
 <https://pytorch.org/docs/master/distributed.html#torch.distributed.init_process_group>`_
 with the desired backend and the rank of the current host.
+Warning: Some torch features, such as (and likely not limited to) ``torch.nn.SyncBatchNorm``
+is not supported and its existence in ``init_process_group`` will cause an exception during
+distributed training.
 
 .. code:: python