[2.0] - Split configuration and modeling files
Refactor to split configuration and modeling files so we can share configuration easily between various frameworks.

This PR is quite annoying to rebase so we should probably merge it pretty soon.
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1201?src=pr&el=h1) Report
> Merging [#1201](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1201?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/0b52642d379bed155e8aa4f4088588bfd8ceaa88?src=pr&el=desc) will **increase** coverage by `0.03%`.
> The diff coverage is `89.57%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1201/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1201?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1201      +/-   ##
==========================================
+ Coverage   80.83%   80.86%   +0.03%     
==========================================
  Files          46       57      +11     
  Lines        7878     8016     +138     
==========================================
+ Hits         6368     6482     +114     
- Misses       1510     1534      +24
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1201?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_transformers/modeling\_auto.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1201/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfYXV0by5weQ==) | `53.94% <100%> (+3.94%)` | :arrow_up: |
| [pytorch\_transformers/tests/modeling\_xlnet\_test.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1201/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdGVzdHMvbW9kZWxpbmdfeGxuZXRfdGVzdC5weQ==) | `95.91% <100%> (+0.02%)` | :arrow_up: |
| [pytorch\_transformers/tests/modeling\_common\_test.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1201/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdGVzdHMvbW9kZWxpbmdfY29tbW9uX3Rlc3QucHk=) | `73.19% <100%> (-4.83%)` | :arrow_down: |
| [pytorch\_transformers/tests/modeling\_gpt2\_test.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1201/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdGVzdHMvbW9kZWxpbmdfZ3B0Ml90ZXN0LnB5) | `93.06% <100%> (+0.06%)` | :arrow_up: |
| [...rch\_transformers/tests/modeling\_distilbert\_test.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1201/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdGVzdHMvbW9kZWxpbmdfZGlzdGlsYmVydF90ZXN0LnB5) | `99.06% <100%> (-0.02%)` | :arrow_down: |
| [pytorch\_transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1201/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfZ3B0Mi5weQ==) | `83.83% <100%> (-0.2%)` | :arrow_down: |
| [pytorch\_transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1201/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfcm9iZXJ0YS5weQ==) | `75.22% <100%> (-0.67%)` | :arrow_down: |
| [pytorch\_transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1201/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfeGxuZXQucHk=) | `77.84% <100%> (-0.99%)` | :arrow_down: |
| [pytorch\_transformers/tests/modeling\_openai\_test.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1201/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdGVzdHMvbW9kZWxpbmdfb3BlbmFpX3Rlc3QucHk=) | `93% <100%> (+0.07%)` | :arrow_up: |
| [pytorch\_transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1201/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfdXRpbHMucHk=) | `90.27% <100%> (+0.2%)` | :arrow_up: |
| ... and [34 more](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1201/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1201?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-transformers/pull/1201?src=pr&el=footer). Last update [0b52642...85df4f7](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1201?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
