Why am I seeing seam lines? (Using Smooth shading, Triangle Mesh)

### 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

As you can see in the figure below, I am getting weird seam lines on certain faces of my 3d model (TriangleMesh object), when using the _smooth shader_. These lines do not seem to appear when I use the _mesh flat shading_. Any idea what might be going on? Any help would be appreciated!