Handle split datasets (train/val//test) in DatasetIO

**Describe the bug**
Input datasets for training tasks could be already split in test, val and test. In that case, there could be necessary to store the paths for each split as it could have some variants in split names.

**Environment:**
 - OS: Windows 11, Linux
 - Python version [3.8, 3.9, 3.10, 3.11]
