Function for computing valid input sizes

[cmalinmayor]

https://github.com/cmalinmayor/linajea_rewrite/blob/765d50628cc9013bf3aef51f8de4e0b2af4864d1/src/linajea/models/detectnet.py#L133

This was one that @neptunes5thmoon and I computed. It might not have all the degrees of freedom, and it has extra stuff about NMS that is not relevant in general, but it's close! Larissa might also have some insight about not needing to crop every time which would allow slightly smaller valid inputs.