token-classification: use is_world_process_zero instead of is_world_master()

[stefan-it]

Hi,

I just found some leftovers of the is_world_master() function in the token classification example.

As this method has been removed, the following error message is thrown when using the do_prediction option:

Traceback (most recent call last):
  File "run_ner.py", line 394, in <module>
    main()
  File "run_ner.py", line 372, in main
    if trainer.is_world_master():
AttributeError: 'Trainer' object has no attribute 'is_world_master'

This PR fixes it and uses the new is_world_process_zero() method instead!

[stefan-it]

/cc @sgugger 🤗

[sgugger]

Thanks for fixing!