fix prediction on run-squad.py example
run_squad.py exits with an error when running do_predict without training.  The error is due to the model_state_dict not existing when --do_predict is selected.

Traceback (most recent call last):
  File "run_squad.py", line 980, in <module>
    main()
  File "run_squad.py", line 923, in main
    model_state_dict = torch.load(output_model_file)
  File "/home/joe/pytorchnlp/lib/python3.5/site-packages/torch/serialization.py", line 365, in load
    f = open(f, 'rb')
FileNotFoundError: [Errno 2] No such file or directory: '../../BERT_work/Squad/pytorch_model.bin'
Thanks @JoeDumoulin 