How to load triangle mesh without bottom plane

### 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 am trying to load the triangle mesh via `o3d.io.read_triangle_mesh(mesh_path)`
The mesh itself is a triangulated obj file, a roof.
When I load the roof as it, I receive an error:
```
[Open3D INFO] Skipping non-triangle primitive geometry of type: 6
mesh TriangleMesh with 0 points and 0 triangles.
```
However, If I add a bottom plane on the roof - it works without any issue.
The question is - how can I load it without the plane?

Please see the image attached - it is the same roof. With plane it can be loaded, without - not.