How to visualize BEV target with point cloud?

### 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 all,

Does Open3D or Open3D-ML support to visualize point cloud with bbox and bird-eye-view(BEV) segmentation target simultaneously? The official tutorials are all about point segmentation, is there any document about pillar semantic?

What i wanna is:
1. maybe there're two visualization layers, one is for bbox labeled vis (like the demo in [README](https://github.com/isl-org/Open3D-ML)), and another one is for bev segmentation target vis
2. how to make bev segmentation target? => split the point cloud frame into grid, maybe 1 grid represents 0.4 meters, hence, the sparse data convert to a dense data representation(like an image), the class label of 1 grid(1 pixel) is decided by the most numerous point clouds category
3. visualize 1. and 2. over one point cloud frame.

If anyone have any solutions, please share your idea, thanks for all. ;-)