Add OSCAR
Continuation of #348 
The files have been moved to S3 and only the unshuffled version is available.
Both original and deduplicated versions of each language are available.

Example of usage:
```python
from datasets import load_dataset

oscar_dedup_en = load_dataset("oscar", "unshuffled_deduplicated_en", split="train")
oscar_orig_fr = load_dataset("oscar", "unshuffled_original_fr", split="train")
```

cc @pjox @jonatasgrosman 

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To make the metadata generation work in parallel I did a few changes in the `datasets-cli test` command to add the `num_proc` and `proc_rank` arguments. This way you can run multiple processes for the metadata computation.

```
datasets-cli test ./datasets/oscar --save_infos --all_configs --num_proc 4 --proc_rank 0 --clear_cache --cache_dir tmp0
```

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ToDo: add the dummy_data