refactor SegmentationModel into subclasses based on backend

Once #41 is merged we should consider refactoring `SegmentationModel` into subclasses.

Currently `SegmentationModel` has a `backend` option based on whether we want to use monai deploy and how we want to use it.
It has branching logic that governs different behaviors based on different choices of backend.
It would be more organized if each backend option were actually a separate class, perhaps subclasses of some abstract SegmentationModel.

Start by reviewing the current SegmentationModel and the changes from issue #41, focusing on its backend option and the branching logic for MONAI Deploy usage. Map the backend-specific behaviors before deciding on subclasses and an abstract SegmentationModel; done means each backend choice has a separate class with the existing behavior preserved.

Written by the indexing model from the issue text.