device mesh doc update

torch/distributed/fsdp/fully_sharded_data_parallel.py

@@ -398,6 +398,14 @@ class FullyShardedDataParallel(nn.Module, _FSDPState):
             ``ignored_modules`` soon. For backward compatibility, we keep both
             ``ignored_states`` and `ignored_modules``, but FSDP only allows one
             of them to be specified as not ``None``.
+        device_mesh (Optional[DeviceMesh]): DeviceMesh can be used as an altenative to
+            process_group. When device_mesh is passed, FSDP will use the underlying process
+            groups for all-gather and reduce-scatter collective communications. Therefore,
+            these two args need to be mutually exclusive. For hybrid sharding strategies such as
+            ``ShardingStrategy.HYBRID_SHARD``, users can pass in a 2D DeviceMesh instead
+            of a tuple of process groups. For 2D FSDP + TP, users are required to pass in
+            device_mesh instead of process_group. For more DeviceMesh info, please visit:
+            https://pytorch.org/tutorials/recipes/distributed_device_mesh.html
     """
 
     def __init__(