Release 0.6.3
Author: thomwolf
Pull request: yes

Preparing release 0.6.3
- adding Bert whole word masking models
- BERTology:
  - add head masking, head pruning and optional output of multi-head attention output gradients
  - output all layers hidden states in GPT/GPT-2
- PyTorch Hub: adding and checking all the models
- various clean-ups and doc/test improvements

Comments:
# [Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/694?src=pr&el=h1) Report
> Merging [#694](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/694?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/commit/80684f6f86c13a89fc1e4feac248ef96b013765c?src=pr&el=desc) will **increase** coverage by `1.17%`.
> The diff coverage is `95.76%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/694/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/694?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master     #694      +/-   ##
==========================================
+ Coverage   67.19%   68.37%   +1.17%     
==========================================
  Files          18       18              
  Lines        3847     3990     +143     
==========================================
+ Hits         2585     2728     +143     
  Misses       1262     1262
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/694?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_pretrained\_bert/modeling\_transfo\_xl.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/694/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvbW9kZWxpbmdfdHJhbnNmb194bC5weQ==) | `59.33% <ø> (ø)` | :arrow_up: |
| [pytorch\_pretrained\_bert/tokenization\_openai.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/694/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvdG9rZW5pemF0aW9uX29wZW5haS5weQ==) | `81.34% <ø> (ø)` | :arrow_up: |
| [pytorch\_pretrained\_bert/tokenization\_transfo\_xl.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/694/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvdG9rZW5pemF0aW9uX3RyYW5zZm9feGwucHk=) | `32.59% <ø> (ø)` | :arrow_up: |
| [pytorch\_pretrained\_bert/tokenization.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/694/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvdG9rZW5pemF0aW9uLnB5) | `91.78% <ø> (-0.92%)` | :arrow_down: |
| [pytorch\_pretrained\_bert/tokenization\_gpt2.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/694/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvdG9rZW5pemF0aW9uX2dwdDIucHk=) | `83.51% <ø> (+1.06%)` | :arrow_up: |
| [pytorch\_pretrained\_bert/modeling\_openai.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/694/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvbW9kZWxpbmdfb3BlbmFpLnB5) | `80.3% <93.65%> (+2%)` | :arrow_up: |
| [pytorch\_pretrained\_bert/modeling\_gpt2.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/694/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvbW9kZWxpbmdfZ3B0Mi5weQ==) | `81.91% <94.93%> (+2.52%)` | :arrow_up: |
| [pytorch\_pretrained\_bert/modeling.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/694/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvbW9kZWxpbmcucHk=) | `89.16% <97.87%> (+0.59%)` | :arrow_up: |
| ... and [2 more](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/694/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/694?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/694?src=pr&el=footer). Last update [80684f6...4447f27](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/694?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).