ddp fixes for training
Author: winglian
Pull request: yes

# What does this PR do?
While trying to train Stable LM or even Llama, I ran into a couple of issues with multi-gpu and DDP. 

I've added a check to skip this for this case since torch doesn't support it: see https://github.com/pytorch/pytorch/blob/main/torch/nn/parallel/distributed.py#L686-L694

```
  File "/opt/conda/lib/python3.8/site-packages/transformers/trainer.py", line 1633, in train                                                                                                                                   
    return inner_training_loop(                                                                                                                                                                                                
  File "/opt/conda/lib/python3.8/site-packages/transformers/trainer.py", line 1720, in _inner_training_loop                                                                                                                    
    model = self._wrap_model(self.model_wrapped)                                                                                                                                                                               
  File "/opt/conda/lib/python3.8/site-packages/transformers/trainer.py", line 1545, in _wrap_model                                                                                                                             
    model = nn.parallel.DistributedDataParallel(                                                                                                                                                                               
  File "/opt/conda/lib/python3.8/site-packages/torch/nn/parallel/distributed.py", line 571, in __init__                                                                                                                            self._log_and_throw(                                                                                                                                                                                                       
  File "/opt/conda/lib/python3.8/site-packages/torch/nn/parallel/distributed.py", line 769, in _log_and_throw                                                                                                                      raise err_type(err_msg)                                                                                                                                                                                                    
RuntimeError: DistributedDataParallel is not needed when a module doesn't have any parameter that requires a gradient.               

```

Added another check for the method `no_sync`

```
  File "/opt/conda/lib/python3.8/site-packages/transformers/trainer.py", line 1634, in train                                                                                                                                   
    return inner_training_loop(                                                                                
  File "/opt/conda/lib/python3.8/site-packages/transformers/trainer.py", line 1900, in _inner_training_loop                                                                                                                        with model.no_sync():                                                                                                                                                                                                      
  File "/opt/conda/lib/python3.8/site-packages/torch/nn/modules/module.py", line 1614, in __getattr__                                                                                                                          
    raise AttributeError("'{}' object has no attribute '{}'".format(                                                                                                                                                           AttributeError: 'GPTNeoXForCausalLM' object has no attribute 'no_sync'      

```
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Comments:
_The documentation is not available anymore as the PR was closed or merged._

cc @sgugger, solution is exactly what we have in Accelerate, and would be a good way to keep it working until the Accelerate integration is fully finished :)