pass langs parameter to certain XLM models
Author: yuvalpinter
Pull request: yes

Adding an argument that specifies the language the SQuAD dataset is in so language-sensitive XLMs (e.g. `xlm-mlm-tlm-xnli15-1024`) don't default to language `0`.
Allows resolution of issue #1799 .

Comments:
This seems to be failing a line length check, but my lines are not the longest in the file -- let me know if I should edit (the whole file) to conform.

Hi, thanks for opening this pull request! For the code quality to pass, you can check what's wrong with `make quality` at the root of the repo, and fix the black/isort issues with `make style`. Do you mind running the latter command and pushing your changes?

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2734?src=pr&el=h1) Report
> Merging [#2734](https://codecov.io/gh/huggingface/transformers/pull/2734?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/9c67196b83a824df577742d32d38e9121d8a9285?src=pr&el=desc) will **decrease** coverage by `1.08%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2734/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2734?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##           master   #2734      +/-   ##
=========================================
- Coverage   74.09%     73%   -1.09%     
=========================================
  Files          93      93              
  Lines       15249   15249              
=========================================
- Hits        11298   11133     -165     
- Misses       3951    4116     +165
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2734?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2734/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9weXRvcmNoX3V0aWxzLnB5) | `8.72% <0%> (-81.21%)` | :arrow_down: |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2734/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `55.39% <0%> (-9.86%)` | :arrow_down: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/2734/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `70.94% <0%> (-2.28%)` | :arrow_down: |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/2734/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `92.07% <0%> (-2.21%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/2734/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `80.06% <0%> (-1.33%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2734?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/transformers/pull/2734?src=pr&el=footer). Last update [9c67196...6070974](https://codecov.io/gh/huggingface/transformers/pull/2734?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Thanks for introducing me to new code check tools! Looks like we're good?

Great, thank you for doing the changes!