KeyError ' '- run_ner.py - Transformers 2.8.0
# 🐛 Bug

## Information

Model I am using (Bert, XLNet ...): BERT

Language I am using the model on (English, Chinese ...): PT

The problem arises when using:
* [ X ] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [ X ] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

I'm having trouble running a dataset in Portuguese. It is in the conll pattern

Traceback (most recent call last):
  File "/home/lucasrodrigues/code/transformers-2.8.0/examples/ner/run_ner.py", line 292, in <module>
    main()
  File "/home/lucasrodrigues/code/transformers-2.8.0/examples/ner/run_ner.py", line 170, in main
    if training_args.do_train
  File "/home/lucasrodrigues/code/transformers-2.8.0/examples/ner/utils_ner.py", line 124, in __init__
    pad_token_label_id=self.pad_token_label_id,
  File "/home/lucasrodrigues/code/transformers-2.8.0/examples/ner/utils_ner.py", line 207, in convert_examples_to_features
    print([label_map[label]] + [pad_token_label_id] * (len(word_tokens) - 1))
KeyError: ''

## Expected behavior

Execution of the NER.

## Environment info
     
- `transformers` version: 2.8.0
- Platform: Linux-4.15.0-76-generic-x86_64-with-debian-buster-sid
- Python version: 3.6.10
- PyTorch version (GPU?): 1.3.1 (True)
- Tensorflow version (GPU?): 2.0.0 (True)
- Using GPU in script?:  <fill in>
- Using distributed or parallel set-up in script?: <fill in>

Have you got two blank lines consecutive? Look at the end of the file, maybe you got an extra blank line or an empty line at the end of the file.<|||||>Additionally, the dataset format must be in a `Token Label` (delimiter is a space) format.

The error message shows, that an unexpected label ( `"`) was found, so I could imagine, that the dataset format is not consistent.

To check this, just do the following on your training, development and test set:

```bash
cat train.txt dev.txt test.txt | grep -v "^$" | cut -d " " -f 2 | sort | uniq
```

This should give you all labels. If you see other tokens, then there's something wrong in the dataset.

<|||||>This problem also happened to me when I trained with my dataset.

Possible causes:
1. There is an extra character in labels
1. The word and label is not separated by space but tab space `\t` <- my main culprit<|||||>Hi can I look into this issue. I am very interested to start contributing in this project<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
