Make all dataset downloads deterministic by applying `sorted` to glob and os.listdir
This PR makes all datasets loading deterministic by applying `sorted()` to all `glob.glob` and `os.listdir` statements.

Are there other "non-deterministic" functions apart from `glob.glob()` and `os.listdir()` that you can think of @thomwolf @lhoestq @mariamabarham @jplu ?

**Important** 
It does break backward compatibility for these datasets because
1. When loading the complete dataset the order in which the examples are saved is different now
2. When loading only part of a split, the examples themselves might be different.

@patrickvonplaten - the nlp / longformer notebook has to be updated since the examples might now be different