Fix config creation for data files with NamedSplit
During config creation, we need to iterate through the data files of all the splits to compute a hash.
To make sure the hash is unique given a certain combination of files/splits, we sort the split names.
However the `NamedSplit` objects can't be passed to `sorted` and currently it raises an error: we need to sort the string of their names instead.

Fix #705 