Query points in neighbor search FixedRadiusSearch

### Checklist

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

### My Question

Hi,
I want to generate a graph from a cloud of points, getting all edges connecting points closer than a given distance. For that, the best option seems to be [torch.layers.FixedRadiusSearch](http://www.open3d.org/docs/release/python_api/open3d.ml.torch.layers.FixedRadiusSearch.html#open3d-ml-torch-layers-fixedradiussearch), which gives `neighbors_index` as the indexes of neighbors. However, it does not output the query points corresponding to these neighbors. The only mention about that appears in the `neighbors_index` documentation:

The compact list of indices of the neighbors. The corresponding query point can be inferred from the ‘neighbor_count_row_splits’ vector.

The mentioned `neighbor_count_row_splits` vector seems to be undefined and does not appear in the documentation.

Is there a way to efficiently obtain the query points from the FixedRadiusSearch output?
Many thanks in advance!