run_xnli doesn't execute
# 🐛 Bug

## Information

(a related issue is this: #3950 )

I am trying to run the `run_xnli.py` script from the [Examples documentation](https://huggingface.co/transformers/examples.html#xnli), but I am getting an error. I am trying to run the script without training.

## To reproduce

Steps to reproduce the behavior:

I follow the [documentation](https://huggingface.co/transformers/examples.html#xnli)

My args are the following:

```
export XNLI_DIR=/path/to/xnli

python run_xnli.py \
  --model_type bert \
  --model_name_or_path bert-base-multilingual-uncased \
  --language de \
  --train_language en \
  --do_eval \
  --data_dir $XNLI_DIR \
  --per_gpu_train_batch_size 32 \
  --learning_rate 5e-5 \
  --num_train_epochs 1.0 \
  --max_seq_length 128 \
  --output_dir bert-base-multilingual-uncased \
  --save_steps -1
```

I get the following error:

```
Traceback (most recent call last):
  File "run_xnli.py", line 646, in <module>
    main()
  File "run_xnli.py", line 638, in main
    result = evaluate(args, model, tokenizer, prefix=prefix)
  File "run_xnli.py", line 290, in evaluate
    outputs = model(**inputs)
  File "/mounts/Users/cisintern/antmarakis/anaconda3/lib/python3.7/site-packages/torch/nn/modules/module.py", line 550, in __call__
    result = self.forward(*input, **kwargs)
  File "/mounts/Users/cisintern/antmarakis/anaconda3/lib/python3.7/site-packages/transformers/modeling_bert.py", line 1139, in forward
    loss = loss_fct(logits.view(-1, self.num_labels), labels.view(-1))
  File "/mounts/Users/cisintern/antmarakis/anaconda3/lib/python3.7/site-packages/torch/nn/modules/module.py", line 550, in __call__
    result = self.forward(*input, **kwargs)
  File "/mounts/Users/cisintern/antmarakis/anaconda3/lib/python3.7/site-packages/torch/nn/modules/loss.py", line 932, in forward
    ignore_index=self.ignore_index, reduction=self.reduction)
  File "/mounts/Users/cisintern/antmarakis/anaconda3/lib/python3.7/site-packages/torch/nn/functional.py", line 2317, in cross_entropy
    return nll_loss(log_softmax(input, 1), target, weight, None, ignore_index, None, reduction)
  File "/mounts/Users/cisintern/antmarakis/anaconda3/lib/python3.7/site-packages/torch/nn/functional.py", line 2115, in nll_loss
    ret = torch._C._nn.nll_loss(input, target, weight, _Reduction.get_enum(reduction), ignore_index)
IndexError: Target 2 is out of bounds.
```

## Expected behavior

I am not sure I am afraid. I am expecting some sort of accuracy measure as shown in the Example.

## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 2.8.0
- Platform: Linux-4.15.0-74-generic-x86_64-with-debian-buster-sid
- Python version: 3.7.6
- PyTorch version (GPU?): 1.5.0 (False)
- Tensorflow version (GPU?): 2.1.0 (False)
- Using GPU in script?:
- Using distributed or parallel set-up in script?:
