How to access the Camera object ?

### 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

Here is my code:

```
vis = o3d.visualization.Visualizer()
vis.create_window()
vis.add_geometry(mesh)
vis.add_geometry(pointcloud)
ctr = vis.get_view_control()
ctr.convert_from_pinhole_camera_parameters(pose, allow_arbitrary=True)
vis.run()
vis.destroy_window()
```

I want to access `open3d.visualization.rendering.Camera` so I can run the method `unproject()`

Is this possible ?
I have found no documentation on show to use the `unproject()` method other than the official one but it does not work.
Here is that I tried:

```
o3d.visualization.rendering.Camera.unproject(120.0, 900.0, 1000.0, 1920.0, 1080.0)
*** TypeError: unproject(): incompatible function arguments. The following argument types are supported:
1. (self: open3d.cuda.pybind.visualization.rendering.Camera, arg0: float, arg1: float, arg2: float, arg3: float, arg4: float) -> numpy.ndarray[numpy.float32[3, 1]]
```