Bug fix in `split_index` method (#5292)
Bug description: on a dataset of 20 samples, when running 4 workers with
8 threads per worker, then the `split_dataset` would return for worker
id `1`:
```
self.worker_splits
[[0, 5], [5, 10], [10, 15], [15, 20]]
self.thread_splits
[[5, 6], [6, 7], [7, 8], [8, 9], [9, 10], [10, 10], [11, 10], [12, 10]]
```
`thread_splits` is wrong and causes a crash in the `DataAnalyzer`: the
end sample id is lower than the initial one on the last 2 threads.
This PR fixes that by fixing the behaviour of `split_index`
---------
Co-authored-by: Logan Adams <114770087+loadams@users.noreply.github.com>
