run_ner.py / bert-base-multilingual-cased can output empty tokens
Author: mfuntowicz
Pull request: yes

This can happen when using bert-base-multilingual-cased with an input containing an unique space.
In this case, the tokenizer will output just an empty word_tokens thus leading to an non-consistent behavior
over the labels_ids tokens adding one more tokens than tokens vector.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2991?src=pr&el=h1) Report
> Merging [#2991](https://codecov.io/gh/huggingface/transformers/pull/2991?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/38f5fe9e0277df67a01db80a1c640ac072a2381e?src=pr&el=desc) will **decrease** coverage by `1.03%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2991/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2991?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2991      +/-   ##
==========================================
- Coverage   77.16%   76.12%   -1.04%     
==========================================
  Files          98       98              
  Lines       15997    15997              
==========================================
- Hits        12344    12178     -166     
- Misses       3653     3819     +166
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2991?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2991/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9weXRvcmNoX3V0aWxzLnB5) | `8.72% <0%> (-81.21%)` | :arrow_down: |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2991/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `85.71% <0%> (-10%)` | :arrow_down: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/2991/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `73.48% <0%> (-2.3%)` | :arrow_down: |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/2991/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `96.03% <0%> (-2.21%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/2991/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `80.2% <0%> (-1.35%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2991/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `92.2% <0%> (-0.17%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2991?src=pr&el=footer). Last update [38f5fe9...57f312d](https://codecov.io/gh/huggingface/transformers/pull/2991?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


This looks good to me. Maybe we can make it a test? This might break some of the other examples as well I will check.