Can't pickle PointCloud or pose_graph in Python multiprocessing

I need to align and stitch together a number of point clouds to generate a 3D model. Using the multiway_registration example as a starting point I am able to generate the 3D model. However I need to speed up the process. I am trying to use Python multiprocessing for speeding up computation of pose_graph. When I try to put the pose_graph (or even a PointCloud) in a multiprocessing.Queue object I get an error “TypeError: can’t pickle open3d.cpu.pybind.geometry.PointCloud objects”. I tried the workaround suggested by for point clouds but the error remains. Can anyone please help out?