Split config weights
Author: thomwolf
Pull request: yes

Split config and weights files for Bert also (was only done for GPT/GPT-2/Transformer-XL.
This will:
- make the Bert model instantiation faster (no need to untar an archive)
- simplify the distributed training (no need to have one archive for each process).

Comments:
# [Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/696?src=pr&el=h1) Report
> Merging [#696](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/696?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/commit/a6f2511811f08c24184f8162f226f252cb6ceaa4?src=pr&el=desc) will **decrease** coverage by `0.13%`.
> The diff coverage is `68.18%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/696/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/696?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master     #696      +/-   ##
==========================================
- Coverage   68.37%   68.23%   -0.14%     
==========================================
  Files          18       18              
  Lines        3990     3976      -14     
==========================================
- Hits         2728     2713      -15     
- Misses       1262     1263       +1
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/696?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_pretrained\_bert/modeling\_openai.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/696/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvbW9kZWxpbmdfb3BlbmFpLnB5) | `80.21% <100%> (-0.09%)` | :arrow_down: |
| [pytorch\_pretrained\_bert/modeling\_gpt2.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/696/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvbW9kZWxpbmdfZ3B0Mi5weQ==) | `81.83% <100%> (-0.08%)` | :arrow_down: |
| [pytorch\_pretrained\_bert/modeling\_transfo\_xl.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/696/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvbW9kZWxpbmdfdHJhbnNmb194bC5weQ==) | `59.21% <100%> (-0.12%)` | :arrow_down: |
| [pytorch\_pretrained\_bert/modeling.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/696/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvbW9kZWxpbmcucHk=) | `88.55% <56.25%> (-0.61%)` | :arrow_down: |
| [pytorch\_pretrained\_bert/tokenization.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/696/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvdG9rZW5pemF0aW9uLnB5) | `92.69% <0%> (+0.91%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/696?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/696?src=pr&el=footer). Last update [a6f2511...f964753](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/696?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).