How to export and visualize a Pose Graph?

### 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 would like to export the pose graph from the Mulitway Registration example, and some how visualize it.
I am stuck on the process of exporting the pose graph, using o3d.io.write_pose_graph(filename, posegraph).
I receive this error: [Open3D WARNING] Write pipelines::registration::PoseGraph failed: unknown file extension. I am unsure of what file extension to set, as the formatting page is missing from the documentation.

Thanks