DataParallel fixes (#5733)

* DataParallel fixes:

1. switched to a more precise check
-        if self.args.n_gpu > 1:
+        if isinstance(model, nn.DataParallel):

2. fix tests - require the same fixup under DataParallel as the training module

* another fix

examples/question-answering/run_squad.py

@@ -199,6 +199,9 @@ def train(args, train_dataset, model, tokenizer):
                         {"langs": (torch.ones(batch[0].shape, dtype=torch.int64) * args.lang_id).to(args.device)}
                     )
 
+            if isinstance(model, torch.nn.DataParallel):
+                inputs["return_tuple"] = True
+
             outputs = model(**inputs)
             # model outputs are always tuple in transformers (see doc)
             loss = outputs[0]