Update run_xnli.py to use Datasets library
# What does this PR do?


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Fixes #9754


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Models:

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Just tested the script locally and it seems to work great, congrats! We are almost done! The last part would be top adapt the end of the README in the text-classification folder to reflect how to use the new script (since the arguments are a bit different).<|||||>> Just tested the script locally and it seems to work great, congrats! We are almost done! The last part would be top adapt the end of the README in the text-classification folder to reflect how to use the new script (since the arguments are a bit different).

I've changed the script in README from
```
export XNLI_DIR=/path/to/XNLI

python run_xnli.py \
  --model_name_or_path bert-base-multilingual-cased \
  --language de \
  --train_language en \
  --do_train \
  --do_eval \
  --data_dir $XNLI_DIR \
  --per_device_train_batch_size 32 \
  --learning_rate 5e-5 \
  --num_train_epochs 2.0 \
  --max_seq_length 128 \
  --output_dir /tmp/debug_xnli/ \
  --save_steps -1
```
to 
```
python run_xnli.py \
  --model_name_or_path bert-base-multilingual-cased \
  --language de \
  --train_language en \
  --do_train \
  --do_eval \
  --per_device_train_batch_size 32 \
  --learning_rate 5e-5 \
  --num_train_epochs 2.0 \
  --max_seq_length 128 \
  --output_dir /tmp/debug_xnli/ \
  --save_steps -1
```

I've also removed these sentences below from [Fine-tuning on XNLI](https://github.com/huggingface/transformers/blob/master/examples/text-classification/README.md#fine-tuning-on-xnli)
```
The data for XNLI can be downloaded with the following links and should be both saved (and un-zipped) in a $XNLI_DIR directory.

    XNLI 1.0
    XNLI-MT 1.0
```<|||||>> This is in good shape to be merged, thanks a lot for your work! I just have a few comments on how to simplify things here and there since there is only one task to deal with in the new script.
> 
> One question I have is, is the tokenizer the same for the training and evaluation datasets, even if the languages, can be different?

I'm puzzled. Is `Trainer()` class doing the magic under the hood when the languages are different? or is it `AutoTokenizer.from_pretrained`?
<|||||>I missed this is a multinlingual checkpoint, so there is no need for different tokenizers.

@patil-suraj it's good to merge IMO, I'll let you review one last time and merge if you approve.<|||||> @sgugger Yay :)
@patil-suraj let me know if there's anything you would like me to change further
<|||||>Thanks @sgugger @patil-suraj  for your helpful comments and guidance. I was jumping in at the deep end when I attempted this PR to be honest, but yay it's merged 😀<|||||>Great job adding this example and thanks a lot for your PR! Don't hesitate to brag a little bit on Twitter about your contribution ;-) 