Added missing code in exemplary notebook - custom datasets fine-tuning
# What does this PR do?

Added missing code in tokenize_and_align_labels function in the exemplary notebook on custom datasets - token classification.
The missing code concerns adding labels for all but the first token in a single word.
The added code was taken directly from huggingface official example - this [colab notebook](https://colab.research.google.com/github/huggingface/notebooks/blob/master/examples/token_classification.ipynb).

Fixes # (issue)

## Before submitting
- [X] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

- maintained examples (not research project or legacy): @sgugger, @patil-suraj


_The documentation is not available anymore as the PR was closed or merged._<|||||>I don't understand what you mean, the notebook has the same code as the example: they are automatically synced at each merge in the Transformers repo.<|||||>You're right. I pasted the wrong link so the comparison made no sense. The link should have been to the colab notebook: 
https://colab.research.google.com/github/huggingface/notebooks/blob/master/examples/token_classification.ipynb

In the colab notebook `tokenize_and_align_labels` has the `else` clause which is missing in the notebook in Github and hence is missing here: https://huggingface.co/docs/transformers/custom_datasets#token-classification-with-wnut-emerging-entities<|||||>Those are two different tutorials, it's normal they have different code. The one in the main documentation is left as simple as possible on purpose.<|||||>In the main documentation, there is a mention about "Only labeling the first token of a given word. Assign -100 to the other subtokens from the same word.". However, it is not the case with the code below, the `tokenize_and_align_labels` function, where not only are the other subtokens assigned with the true labels but also `previous_word_idx` is not being updated. Such contradiction was ambiguous for me. Only after having dug deeper (the colab notebook), I understood that this part of code was missing from the official documentation. I do not think that these few lines were omitted on purpose. If you don't think it makes such a difference, close this pr (or let me know if I should be the one to close it).<|||||>I adjusted your comments. I guess I could have been more precise from the beginning. Thanks.