DistilBERT for token classification
Hi,

this PR adds a `DistilBertForTokenClassification` implementation (mainly inspired by the BERT implementation) that allows to perform sequence labeling tasks like NER or PoS tagging.

Additionally, the `run_ner.py` example script was modified to fully support DistilBERT for NER tasks.

I did a small comparison between BERT (large, cased), RoBERTa (large, cased) and DistilBERT (base, uncased) with the same hyperparameters as specified in the [example documentation](https://huggingface.co/transformers/examples.html#named-entity-recognition) (one run):

| Model | F-Score Dev | F-Score Test
| --------------------------------- | ------- | --------
| `bert-large-cased`            | 95.59 | 91.70
| `roberta-large`                  | 95.96 | 91.87
| `distilbert-base-uncased` | 94.34 | 90.32

Unit test for the `DistilBertForTokenClassification` implementation is also added.
This is great, thanks @stefan-it!<|||||>This is great, thanks a lot @stefan-it.
I've added your quick benchmark in the readme.<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1792?src=pr&el=h1) Report
> Merging [#1792](https://codecov.io/gh/huggingface/transformers/pull/1792?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/b5d330d11820f4ac2cc8c909b1a6a77e0cd961e0?src=pr&el=desc) will **increase** coverage by `0.03%`.
> The diff coverage is `97.29%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1792/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1792?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1792      +/-   ##
==========================================
+ Coverage   84.03%   84.07%   +0.03%     
==========================================
  Files          94       94              
  Lines       14032    14069      +37     
==========================================
+ Hits        11792    11828      +36     
- Misses       2240     2241       +1
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1792?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/tests/modeling\_distilbert\_test.py](https://codecov.io/gh/huggingface/transformers/pull/1792/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX2Rpc3RpbGJlcnRfdGVzdC5weQ==) | `99.18% <100%> (+0.08%)` | :arrow_up: |
| [transformers/modeling\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/1792/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2Rpc3RpbGJlcnQucHk=) | `95.87% <96.15%> (+0.02%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/1792?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1792?src=pr&el=footer). Last update [b5d330d...05db5bc](https://codecov.io/gh/huggingface/transformers/pull/1792?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>I just wanted to say thanks for this PR, it was just what I was looking for at the time.