How to perform ray-voxel intersection in Open3D on generic voxel grids?

### 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](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 have a voxel grid built from a point cloud and a set of cameras. I would like to check for all the camera rays which voxels they intersect. This doesn't sound too hard to do from scratch, but I'm wondering if it can be done with existing algorithms in Open3D. I know that [this](http://www.open3d.org/docs/latest/python_api/open3d.t.geometry.VoxelBlockGrid.html#open3d.t.geometry.VoxelBlockGrid.ray_cast) function exist, but it needs a TSDF, while [this](http://www.open3d.org/docs/latest/python_api/open3d.t.geometry.RaycastingScene.html) class is designed for meshes. Can we do the same thing for generic voxels using current features? Thank you in advance for the help.