update xsum
Looks cool!
@mariamabarham can you add a detailed description here what exactly is changed and how the user can load xsum now? And a rebase should solve the conflicts This is a super useful PR :-) @sshleifer - maybe you can take a look at the updated version of xsum if you can use it for your use case. Now, one should be able to just load it with:

```python 
nlp.load_datasets("xsum", ....)  # no manual dir required anymore
```

This PR makes the following update to the xsum dataset:

- Manual download is not required anymore

- dataset can be loaded as follow: `nlp.load_dataset('xsum')`


**Important** 
Instead of using on outdated url to download the data:  "https://raw.githubusercontent.com/EdinburghNLP/XSum/master/XSum-Dataset/XSum-TRAINING-DEV-TEST-SPLIT-90-5-5.json" 

a more up-to-date url stored here: https://s3.amazonaws.com/datasets.huggingface.co/summarization/xsum.tar.gz is used
, so that the user does not need to manually download the data anymore. 
There might be slight breaking changes here for xsum. 