Add interleave_datasets for map-style datasets
### Add interleave_datasets for map-style datasets

Add support for map-style datasets (i.e. `Dataset` objects) in `interleave_datasets`.
It was only supporting iterable datasets (i.e. `IterableDataset` objects).

### Implementation details

It works by concatenating the datasets and then re-order the indices to make the new dataset.

### TODO
- [x] tests
- [x] docs

Close #2563 