Random model initialization doesn't work

### 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

So I was just exploring stuff and tried to train tf PointPillars model as in the tutorial. I didn't set any arguments as the tutorial said that they would initialize randomly.

`model = ml3d.models.PointPillars()`

But I get the following error:

> TypeError: __init__() missing 1 required positional argument: 'voxel_size'

I tried setting the voxel_size manually but it didn't work either.