o3d.io.read_point_cloud can't get the point label.

### 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

When I used o3d.io.read_point_cloud API to read a pcd file into code, i found only the point xyz info but not label into software. How can i extract label into software which is used to color.