XLM-RoBERTa model for QA seems not properly work
## Environment info
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     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 3.3.1 (and I've also tried installing Transformers from `master`, see details below)
- Platform: Linux-4.19.112+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.6.9
- PyTorch version (GPU?): 1.6.0+cu101 (False)
- Tensorflow version (GPU?): 2.3.0 (False)
- Using GPU in script?: Yes
- Using distributed or parallel set-up in script?: No

### Who can help
albert, bert, GPT2, XLM: @LysandreJik

## Information

Model I am using (Bert, XLNet ...): 
[deepset/xlm-roberta-large-squad2](https://huggingface.co/deepset/xlm-roberta-large-squad2) 

The problem arises when using:
* [x] the official example scripts: [run_squad.py](https://github.com/huggingface/transformers/blob/master/examples/question-answering/run_squad.py)
* [ ] my own modified scripts: (give details below)

The tasks I am working on is:
* [x] an official GLUE/SQUaD task: **SQuAD 2.0 dev set evaluation**
* [ ] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:
```
! wget https://raw.githubusercontent.com/rajpurkar/SQuAD-explorer/master/dataset/dev-v2.0.json
! python transformers/examples/question-answering/run_squad.py \
  --model_type xlm-roberta \
  --model_name_or_path 'deepset/xlm-roberta-large-squad2' \
  --do_eval \
  --do_lower_case \
  --predict_file 'dev-v2.0.json' \
  --output_dir 'output' \
  --overwrite_output_dir \
  --version_2_with_negative
```

## Expected behavior
There are some values mismatch between:
1. values reported in the model card [here](https://huggingface.co/deepset/xlm-roberta-large-squad2#performance)
2. values obtained when Transformers is installed using `pip install transformers` 
3. values obtained when Transformers is installed from master

In particular:
- Reported metrics in the model card: `"exact": 79.45759285774446, "f1": 83.79259828925511`
- Transformers installed from pip: `'exact': 64.67615598416576, 'f1': 77.27580544355429`
- Transformers installed from master: `'exact': 60.11959090373114, 'f1': 76.13129575803934`

Thanks for reporting, will investigate.<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
<|||||>hello, this command "  --model_type xlm-roberta \" not work for me,
can you help me? please