Is it possible to get face orientation for each triangle?

### 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

I am trying to calculate normals for each triangle based on its face orientation, and not based on points normals. For this I would need face orientation for each triangle. Is this possible to obtain? I know that face orientations are calculated, but I am not if they can be accessed.