What does the return values of voxel_down_sample_and_trace() mean?

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](https://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](https://www.open3d.org/docs/release/) and the [latest documentation](https://www.open3d.org/docs/latest/) (for `main` branch).

### My Question

I am very confused about the return values meanings.
Here I run the following code:

```python
result_pcd, cube_ids, original_indices = pcd.voxel_down_sample_and_trace(RESOLUTION, min_bound, max_bound)
```
And I got these printing results and just want to know what the 8 numbers mean in each row of `cube_ids`? Why there are `4` point indices in `original_indices[0]`? It seems the API of this function is different from before when the function only returns 2 values.