Load Mesh error: Read a mesh with .obj format

### 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 use open3d to load a mesh (.obj) from disk, but result a warning and no data loaded, here is the code :

`o3d.io.read_triangle_mesh('2.obj')`

here is the warning message:

> [Open3D INFO] Skipping non-triangle primitive geometry of type: 20

BTW, I can load this obj in other software, such as Blender or Meshroom, this open3d version is 0.15.1