Add integration tests for xlm roberta modelling and xlm roberta tokenzier
It's quite easy to get real numbers for the XLM-R moder from [fairseq](https://github.com/pytorch/fairseq/tree/master/examples/xlmr), so I added integration tests for `xlm_roberta_modeling.py` and `xlm_roberta_tokenization.py` 

Since `XLMRobertaModel` is the same model as `RobertaModel`, I think intergration tests for `xlm_roberta_modeling.py` are enough. 
Regarding `XLMRobertaTokenizer` there were no tests so far, so in this file there should definitely be more "fast" tests. I would need some help on those (@mfuntowicz ?). 

Regarding the results of the integration tests:

The tests for `xlm_roberta_modeling.py` all pass. 
One of the two tests (called hard_token_symbols) for `xlm_roberta_tokenization.py' fails. @LysandreJik @mfuntowicz 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3014?src=pr&el=h1) Report
> Merging [#3014](https://codecov.io/gh/huggingface/transformers/pull/3014?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e693cd1e877aa191d3317faed33e87d1558c9406?src=pr&el=desc) will **decrease** coverage by `1.03%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3014/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/3014?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3014      +/-   ##
==========================================
- Coverage   77.25%   76.22%   -1.04%     
==========================================
  Files          98       98              
  Lines       16040    16040              
==========================================
- Hits        12392    12226     -166     
- Misses       3648     3814     +166
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3014?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3014/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/3014/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `85.71% <0%> (-10%)` | :arrow_down: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/3014/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `73.48% <0%> (-2.3%)` | :arrow_down: |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/3014/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `96.03% <0%> (-2.21%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/3014/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `80.2% <0%> (-1.35%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3014/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `92.22% <0%> (-0.17%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3014?src=pr&el=footer). Last update [e693cd1...fc5fe85](https://codecov.io/gh/huggingface/transformers/pull/3014?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
