Updating tests and doc
Author: thomwolf
Pull request: yes

- Fix GPT-2 test
- Update the documentation

Comments:
# [Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/687?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@cad88e1`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/687/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/687?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master     #687   +/-   ##
=========================================
  Coverage          ?   67.14%           
=========================================
  Files             ?       18           
  Lines             ?     3847           
  Branches          ?        0           
=========================================
  Hits              ?     2583           
  Misses            ?     1264           
  Partials          ?        0
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/687?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_pretrained\_bert/modeling\_gpt2.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/687/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvbW9kZWxpbmdfZ3B0Mi5weQ==) | `79.39% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/687?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/687?src=pr&el=footer). Last update [cad88e1...44e9ddd](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/687?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


# [Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/687?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@cad88e1`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/687/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/687?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master     #687   +/-   ##
=========================================
  Coverage          ?   67.14%           
=========================================
  Files             ?       18           
  Lines             ?     3847           
  Branches          ?        0           
=========================================
  Hits              ?     2583           
  Misses            ?     1264           
  Partials          ?        0
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/687?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_pretrained\_bert/modeling\_gpt2.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/687/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvbW9kZWxpbmdfZ3B0Mi5weQ==) | `79.39% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/687?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/687?src=pr&el=footer). Last update [cad88e1...44e9ddd](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/687?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).