Updating examples
This PR check that the examples are working well (fix learning rate bug in distributed settings)
Also:
- prepare 2 fine-tuned models on SQuAD (BERT Whole Word Masking) so people can also use fine-tuned models (nice performances: "exact_match": 86.9, "f1": 93.2, better than the original Google AI values)
- add a bertology script which showcases:
  * computing head attention entropy
  * computing head importance scores according to http://arxiv.org/abs/1905.10650
  * performing head masking and head pruning (like masking but you actually remove the weights) according to http://arxiv.org/abs/1905.10650
# [Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/697?src=pr&el=h1) Report
> Merging [#697](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/697?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/commit/3763f8944dc3fef8afb0c525a2ced8a04889c14f?src=pr&el=desc) will **decrease** coverage by `6%`.
> The diff coverage is `62.5%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/697/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/697?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master     #697      +/-   ##
==========================================
- Coverage   68.23%   62.22%   -6.01%     
==========================================
  Files          18       18              
  Lines        3976     3979       +3     
==========================================
- Hits         2713     2476     -237     
- Misses       1263     1503     +240
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/697?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_pretrained\_bert/tokenization.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/697/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvdG9rZW5pemF0aW9uLnB5) | `89.04% <ø> (-3.66%)` | :arrow_down: |
| [pytorch\_pretrained\_bert/modeling\_openai.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/697/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvbW9kZWxpbmdfb3BlbmFpLnB5) | `68.11% <50%> (-12.11%)` | :arrow_down: |
| [pytorch\_pretrained\_bert/modeling.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/697/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvbW9kZWxpbmcucHk=) | `79.49% <66.66%> (-9.06%)` | :arrow_down: |
| [pytorch\_pretrained\_bert/modeling\_gpt2.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/697/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvbW9kZWxpbmdfZ3B0Mi5weQ==) | `69.79% <66.66%> (-12.05%)` | :arrow_down: |
| [pytorch\_pretrained\_bert/modeling\_transfo\_xl.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/697/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvbW9kZWxpbmdfdHJhbnNmb194bC5weQ==) | `52.93% <0%> (-6.29%)` | :arrow_down: |
| [pytorch\_pretrained\_bert/tokenization\_openai.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/697/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvdG9rZW5pemF0aW9uX29wZW5haS5weQ==) | `75.64% <0%> (-5.7%)` | :arrow_down: |
| ... and [2 more](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/697/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/697?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/697?src=pr&el=footer). Last update [3763f89...411981a](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/697?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
