Improvements to GPT-2 (special_tokens, fine-tuning, medium model) + repo code coverage metric
- adding method to add special tokens to GPT-2 (like it's done for GPT).
- adding code coverage tracking for tests.
# [Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/560?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@b832d5b`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `70.37%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/560/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/560?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master     #560   +/-   ##
=========================================
  Coverage          ?   66.04%           
=========================================
  Files             ?       18           
  Lines             ?     3673           
  Branches          ?        0           
=========================================
  Hits              ?     2426           
  Misses            ?     1247           
  Partials          ?        0
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/560?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_pretrained\_bert/modeling\_openai.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/560/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvbW9kZWxpbmdfb3BlbmFpLnB5) | `79.68% <100%> (ø)` | |
| [pytorch\_pretrained\_bert/tokenization\_gpt2.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/560/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvdG9rZW5pemF0aW9uX2dwdDIucHk=) | `84.86% <100%> (ø)` | |
| [pytorch\_pretrained\_bert/modeling\_gpt2.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/560/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvbW9kZWxpbmdfZ3B0Mi5weQ==) | `80.16% <61.9%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/560?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/pytorch-pretrained-BERT/pull/560?src=pr&el=footer). Last update [b832d5b...db98a4a](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/560?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
