Fix spurious label column when directories match split names

[neha222222]

Issue - #7880

[lhoestq]

Great ! I think it works great in general but can have weird edge cases, like if the splits are defined with directories that are not the split names, e.g.

- split: train
  path: "my-dir/*.png"
- split: test
  path: "my-other-dir/*.png"

maybe a more robust way could be to check whether each split has exactly one label, and that the labels of every split are different. wdyt ?