Slice splits support for datasets.load_from_disk

### Feature request

Support for slice splits in `datasets.load_from_disk`, similar to how it's already supported for `datasets.load_dataset`.

### Motivation

Slice splits are convienient in a numer of cases - adding support to `datasets.load_from_disk` would make working with local datasets easier and homogenize the APIs of load_from_disk and load_dataset.

### Your contribution

Sure, if the devs think the feature request is sensible.