[minor] OSS doc fix - add the DDP wrap (#131)

* wrapping the model in DDP in the tutorial

* typo

docs/source/tutorials/oss.rst

 Optimizer state sharding
 ========================
 
-Using torch.nn.parallel.DistributedDataParallel leads to some wasted communications, but it is possible and makes OSS a drop in solution in your existing torch distributed code.
+Using torch.nn.parallel.DistributedDataParallel leads to some wasted communications in the case of OSS, but it is possible and makes OSS a drop in solution in your existing torch distributed code.
 Let's suppose that your trainer looks like
 
 .. code-block:: python
 
 
     import torch
+    from torch.nn.parallel import DistributedDataParallel as DDP
+
 
     def train(
         rank: int,
@@ -19,11 +21,12 @@ Let's suppose that your trainer looks like
 
         # Problem statement
         model = myAwesomeModel().to(rank)
+        model = DDP(model, device_ids=[rank])
         dataloader = mySuperFastDataloader()
         loss_ln = myVeryRelevantLoss()
 
         # optimizer specific arguments e.g. LR, momentum, etc...
-        base_optimizer_arguments = { "lr": 1e-4} 
+        base_optimizer_arguments = { "lr": 1e-4}
         optimizer = torch.optim.SGD(
             params=model.parameters(),
             **base_optimizer_arguments)
@@ -50,6 +53,7 @@ Then sharding the optimizer state is merely a matter of wrapping your optimizer
 
     import torch
     from fairscale.optim.oss import OSS
+    from torch.nn.parallel import DistributedDataParallel as DDP
 
     def train(
         rank: int,
@@ -61,11 +65,12 @@ Then sharding the optimizer state is merely a matter of wrapping your optimizer
 
         # Problem statement
         model = myAwesomeModel().to(rank)
+        model = DDP(model, device_ids=[rank])
         dataloader = mySuperFastDataloader()
         loss_ln = myVeryRelevantLoss()
 
         # optimizer specific arguments e.g. LR, momentum, etc...
-        base_optimizer_arguments = { "lr": 1e-4} 
+        base_optimizer_arguments = { "lr": 1e-4}
 
         # ** NEW ** Wrap a base optimizer into OSS
         base_optimizer = torch.optim.SGD  # any pytorch compliant optimizer
@@ -100,5 +105,5 @@ The above `train` function will then need to be run via a `multiprocessing.spawn
             nprocs=WORLD_SIZE,
             join=True
         )
-    
+
 to see it in action, you can test it with the following script _`tutorial_oss.py <../../../examples/tutorial_oss.py>`_