How to transform specific points/vertices and removing the corresponding stretched triangles

### 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 have a scene with several instances. And I wanted to move some of the instances within the scene. However the translate and transform function apply to all points/vertices in the *.ply file. And If I'm transforming the vertices, the corresponding triangles would be stretched.

For example, before the transformation:
After the transformation:

We use the following code to change the coordinates of the vertices:
`np.asarray(mesh.vertices)[i]+=np.asarray([0.4,1,0])`

Is there some way that we can do this transformation while keeping the triangles normal？