[Feature request] separate split name and split instructions
Currently, the name of an nlp.NamedSplit is parsed in arrow_reader.py and used as the instruction.

This makes it impossible to have several training sets, which can occur when:
- A dataset corresponds to a collection of sub-datasets
- A dataset was built in stages, adding new examples at each stage

Would it be possible to have two separate fields in the Split class, a name /instruction and a unique ID that is used as the key in the builder's split_dict ?