run_ner.py crashes when dev or test contain previously unseen labels
## Environment info

- `transformers` version: 4.2.0dev0
- Platform:  Linux-3.10.0-1127.19.1.el7.x86_64-x86_64-with-centos-7.8.2003-Core
- Python version: 3.6.8
- PyTorch version (GPU?): 1.7.1+cu110 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: yes
- Using distributed or parallel set-up in script?: don't know

### Who can help
@stefan-it

## Information

Model I am using (Bert, XLNet ...): KB/bert-base-swedish-cased

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)

I am using run_ner.py to POS-tag a Swedish corpus (TalbankenSBX). If dev and/or test files contain a label that is not present in the train file, the script crashes. The same issue arises with any other corpus I try.

## To reproduce

Steps to reproduce the behavior:

1. Create two toy files with the following contents:

sbx-1-train.json:
```
{"words": ["Vem", "får", "rösta", "?"], "pos": ["HP.UTR.SIN.IND", "VB.PRS.AKT", "VB.INF.AKT", "MAD"]}
```

sbx-1-dev.json
```
{"words": ["är", "född", "1950", "eller", "tidigare", ","], "pos": ["VB.PRS.AKT", "PC.PRF.UTR.SIN.IND.NOM", "RG.NOM", "KN", "AB.KOM", "MID"]}
```

2. Run  python run_ner.py --model_name_or_path KB/bert-base-swedish-cased --train_file sbx-1-train.json --validation_file sbx-1-dev.json --output_dir sbx1 --do_train --do_eval

This results in: 
```
Traceback (most recent call last):
  File "run_ner.py", line 412, in <module>
    main()
  File "run_ner.py", line 303, in main
    load_from_cache_file=not data_args.overwrite_cache,
  File "/home/sasha/venvs/hugtrans/lib64/python3.6/site-packages/datasets/dataset_dict.py", line 303, in map
    for k, dataset in self.items()
  File "/home/sasha/venvs/hugtrans/lib64/python3.6/site-packages/datasets/dataset_dict.py", line 303, in <dictcomp>
    for k, dataset in self.items()
  File "/home/sasha/venvs/hugtrans/lib64/python3.6/site-packages/datasets/arrow_dataset.py", line 1240, in map
    update_data = does_function_return_dict(test_inputs, test_indices)
  File "/home/sasha/venvs/hugtrans/lib64/python3.6/site-packages/datasets/arrow_dataset.py", line 1211, in does_function_return_dict
    function(*fn_args, indices, **fn_kwargs) if with_indices else function(*fn_args, **fn_kwargs)
  File "run_ner.py", line 288, in tokenize_and_align_labels
    label_ids.append(label_to_id[label[word_idx]])
KeyError: 'PC.PRF.UTR.SIN.IND.NOM'
```

...where PC.PRF.UTR.SIN.IND.NOM is the tag which is not present in the train set. 

## Expected behavior
The script should not crash when encountering unseen tags.
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