Use full released xsum dataset
@lhoestq I took a shot at addressing your comments but the build scripts seem to be complaining about not being able to open dummy files. How do I resolve those errors without copying the full dataset into the dummy dir? Could you check that the names of the dummy data files are right ?
You can use 
```
datasets-cli dummy_data ./datasets/xum
```
to print the expected file names Ok @lhoestq looks like I got the tests to pass :)
#672 Fix xsum to expand coverage and include IDs
Code based on parser from older version of `datasets/xsum/xsum.py`
@lhoestq 