Fix run_mlm_wwm example

[changjonathanc]

What does this PR do?

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

[changjonathanc]

Wait, the example for run_mlm_wwm.py doesn't seem to run:

Traceback (most recent call last):
  File "run_mlm_wwm.py", line 334, in <module>
    main()
  File "run_mlm_wwm.py", line 161, in main
    model_args, data_args, training_args = parser.parse_args_into_dataclasses()
  File ".../transformers/src/transformers/hf_argparser.py", line 144, in parse_args_into_dataclasses
    raise ValueError(f"Some specified arguments are not used by the HfArgumentParser: {remaining_args}")
ValueError: Some specified arguments are not used by the HfArgumentParser: ['--dataset_name', 'wikitext', '--dataset_config_name', 'wikitext-2-raw-v1']

I will try to fix it too.

[sgugger]

Thanks for the fixes. I'm unsure the script can run on datasets hosted on the hub because of the need for reference files, that's why I hadn't added the dataset_name and dataset_config_name arguments in this one.

[changjonathanc]

You can still ignore the dataset_name arg and use the old behavior, so adding dataset_name should be fine.
I want to mention that I did not run the example with ref file.

[sgugger]

Ok, then we should add a check in the post_init to make sure no reference file is passed if we're using dataset_name, otherwise it won't work,

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