audio folder builder cannot detect custom split name

[phineas-pta]

Describe the bug

when using audio folder builder (load_dataset("audiofolder", data_dir="/path/to/folder")), it cannot detect custom split name other than train/validation/test

Steps to reproduce the bug

i have the following folder structure

my_dataset/
├── train/
│   ├── lorem.wav
│   ├── …
│   └── metadata.csv
├── test/
│   ├── ipsum.wav
│   ├── …
│   └── metadata.csv
├── validation/
│   ├── dolor.wav
│   ├── …
│   └── metadata.csv
└── custom/
    ├── sit.wav
    ├── …
    └── metadata.csv

using ds = load_dataset("audiofolder", data_dir="/path/to/my_dataset")

Expected behavior

i got ds with only 3 splits train/validation/test, whenever i rename train/validation/test folder it also disappear if i re-create ds

Environment info

• datasets version: 3.5.0
• Platform: Windows-11-10.0.26100-SP0
• Python version: 3.12.8
• huggingface_hub version: 0.30.2
• PyArrow version: 18.1.0
• Pandas version: 2.2.3
• fsspec version: 2024.9.0