[2.0] Reodering arguments for torch jit #1010 and future TF2.0 compatibility
Torch jit (cf #1010) and TF 2.0 (cf #1104) are more strict than PyTorch on having a specific order of arguments for easy use.

This PR refactor the order of the keyword arguments to make them as natural as possible.

This will be a breaking change for people using positional order to input keyword arguments in the forward pass of the models, hence is delayed to the 2.0 release.
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1195?src=pr&el=h1) Report
> Merging [#1195](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1195?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/0b52642d379bed155e8aa4f4088588bfd8ceaa88?src=pr&el=desc) will **decrease** coverage by `0.4%`.
> The diff coverage is `86.59%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1195/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1195?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1195      +/-   ##
==========================================
- Coverage   80.83%   80.42%   -0.41%     
==========================================
  Files          46       46              
  Lines        7878     7892      +14     
==========================================
- Hits         6368     6347      -21     
- Misses       1510     1545      +35
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1195?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1195/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfeGxuZXQucHk=) | `78.83% <100%> (ø)` | :arrow_up: |
| [pytorch\_transformers/tests/modeling\_bert\_test.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1195/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdGVzdHMvbW9kZWxpbmdfYmVydF90ZXN0LnB5) | `96.29% <100%> (ø)` | :arrow_up: |
| [...rch\_transformers/tests/modeling\_distilbert\_test.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1195/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdGVzdHMvbW9kZWxpbmdfZGlzdGlsYmVydF90ZXN0LnB5) | `99.08% <100%> (ø)` | :arrow_up: |
| [pytorch\_transformers/modeling\_distilbert.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1195/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfZGlzdGlsYmVydC5weQ==) | `96.77% <100%> (ø)` | :arrow_up: |
| [pytorch\_transformers/modeling\_xlm.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1195/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfeGxtLnB5) | `87.08% <100%> (ø)` | :arrow_up: |
| [pytorch\_transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1195/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfYmVydC5weQ==) | `88.03% <100%> (ø)` | :arrow_up: |
| [...ytorch\_transformers/tests/modeling\_roberta\_test.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1195/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdGVzdHMvbW9kZWxpbmdfcm9iZXJ0YV90ZXN0LnB5) | `78.81% <100%> (ø)` | :arrow_up: |
| [pytorch\_transformers/modeling\_transfo\_xl.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1195/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfdHJhbnNmb194bC5weQ==) | `56.9% <42.85%> (-0.1%)` | :arrow_down: |
| [pytorch\_transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1195/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfb3BlbmFpLnB5) | `81.08% <76.47%> (-0.88%)` | :arrow_down: |
| [pytorch\_transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1195/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfZ3B0Mi5weQ==) | `83.13% <76.47%> (-0.91%)` | :arrow_down: |
| ... and [8 more](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1195/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1195?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/1195?src=pr&el=footer). Last update [0b52642...7fba47b](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1195?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Ok merging