refactor: move the vertex specific ModelInput to its corresponding file

What does this merge request do and why?

This class is only used by Vertex classes. I had the option of either:

1. rename it to VertexModelInput and keep it in base.py
2. (chosen) move it into vertex_text.py file

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.