Add MultiEURLEX dataset
Hi @lhoestq, we have this new cool multilingual dataset coming at EMNLP 2021. It would be really nice if we could have it in Hugging Face asap. Thanks!  Hi @lhoestq, I adopted most of your suggestions:

- Dummy data files reduced, including the 2 smallest documents per subset JSONL.
- README was updated with the publication URL and instructions on how to download and use label descriptors. Excessive newlines were deleted.

I would prefer to keep the label list in a pure format (original ids), to enable people to combine those with more information or possibly in the future explore the dataset, find inconsistencies and fix those to release a new version.  Thanks for the changes :)

Regarding the labels:

If you use the ClassLabel feature type, the only change is that it will store the ids as integers instead of (currently) string.
The advantage is that if people want to know what id corresponds to which label name, they can use `classlabel.int2str`. It is also the format that helps automate model training for classification in `transformers`.

Let me know if that sounds good to you or if you still want to stick with the labels as they are now. Hey @lhoestq, thanks for providing this information. This sounds great. I updated my code accordingly to use `ClassLabel`. Could you please provide a minimal example of how `classlabel.int2str` works in practice in my case, where labels are a sequence?

```python
from datasets import load_dataset
dataset = load_dataset('multi_eurlex', 'all_languages')
# Read strs from the labels (list of integers) for the 1st sample of the training split
```

I would like to include this in the README file.

Could you also provide some info on how I could define the supervized key to automate model training, as you said?

Thanks! Thanks for the update :)

Here is an example of usage:
```python
from datasets import load_dataset
dataset = load_dataset('multi_eurlex', 'all_languages', split='train')
classlabel = dataset.features["labels"].feature
print(dataset[0]["labels"])
# [1, 20, 7, 3, 0]
print(classlabel.int2str(dataset[0]["labels"]))
# ['100160', '100155', '100158', '100147', '100149']
```

The ClassLabel is simply used to define the `id2label` dictionary of classification models, to make the ids match between the model and the dataset. There nothing more to do :p 

I think one last thing to do is just update the `dataset_infos.json` file and we'll be good ! Everything is ready! 👍 

**Add new MultiEURLEX Dataset**

MultiEURLEX comprises 65k EU laws in 23 official EU languages (some low-ish resource). Each EU law has been annotated with EUROVOC concepts (labels) by the Publication Office of EU. As with the English EURLEX, the goal is to predict the relevant EUROVOC concepts (labels); this is multi-label classification task (given the text, predict multiple labels).