start_position=0 in utils_squad.py when span is impossible
Hi, 

https://github.com/huggingface/pytorch-transformers/blob/e768f2322abd2a2f60a3a6d64a6a94c2d957fe89/examples/utils_squad.py#L340-L351

when answer is out of span, start_position should be cls_index rather than 0 as L350


And in

https://github.com/huggingface/pytorch-transformers/blob/e768f2322abd2a2f60a3a6d64a6a94c2d957fe89/examples/run_squad.py#L253-L259

when using multi gpu and set evaluate_during_training=True, we may add 
`model_tmp = model.module if hasattr(model, 'module') else model` in order to get `model_tmp.config. start_n_top` rather than `model.config.start_n_top`