attention_mask at run_squad.py
I think there's minor mistake in [run_squad.py](https://github.com/huggingface/pytorch-transformers/blob/5fe0b378d8/examples/run_squad.py#L298) at line 298
```
 inputs = {'input_ids': batch[0], 
'token_type_ids':  None if args.model_type == 'xlm' else batch[1],  
'attention_mask':  batch[2],
'start_positions': batch[3],
'end_positions':   batch[4]}
```
but i think batch[1] is attention_mask and batch[2] is segment_ids, so it should be like this
``` 
inputs = {'input_ids': batch[0],
'attention_mask':  batch[1], 
'token_type_ids':  None if args.model_type == 'xlm' else batch[2],  
'start_positions': batch[3], 
'end_positions':   batch[4]}
```
because the data is 
```
dataset = TensorDataset(all_input_ids, all_input_mask, all_segment_ids,
                                all_start_positions, all_end_positions,
                                all_cls_index, all_p_mask)

```

I have same issue. <|||||>Thanks @seanie12!