Fix examples/run_tf_ner.py label encoding error #2559 
This is an explanation and a proposed fix for #2559 
The code set `pad_token_label_id = 0`, and increase the total number of labels `num_labels = len(labels) + 1`, but made no change to the label list. Thus the first label in label list has the same index as pad_token_label_id.

Following instructions in README take GermEval 2014 as an example, for one sentence in test dataset the token `Aachen` is labeled as `B-LOC` (`B-LOC` is the first label in label list), yet because of the collision with pad_token_label_id, both pad tokens and `Aachen` are encoded as 0:
![image](https://user-images.githubusercontent.com/1331543/72657322-bb90fa00-3957-11ea-980d-33ac12e54545.png)
And the test_predictions.txt is also off by one:
```
1951 I-PERpart
bis I-PERpart
1953 I-PERpart
wurde I-PERpart
...
```

The fix adds a placeholder label `[PAD]` at position 0 when loading the datasets and all labels positions are shifted by 1. The resulting encoding for the same sample sentence:
![image](https://user-images.githubusercontent.com/1331543/72657627-5e974300-395b-11ea-905e-4a3028f4b5c4.png)

And the test_predictions.txt thus has correct index:
```
1951 O
bis O
1953 O
wurde O
...
```

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2575?src=pr&el=h1) Report
> Merging [#2575](https://codecov.io/gh/huggingface/transformers/pull/2575?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/1a8e87be4e2a1b551175bd6f0f749f3d2289010f?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2575/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2575?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2575   +/-   ##
=======================================
  Coverage   74.53%   74.53%           
=======================================
  Files          87       87           
  Lines       14819    14819           
=======================================
  Hits        11046    11046           
  Misses       3773     3773
```



------

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