Why not add a new abstraction for different network topologies

[Alicegaz]

Suggestions are to rename the models package to nn_components (or something more appropriate) and add a new package named topologies. Topologies will store classes for constructs of backbone, pooling, neck, and head, with different classes for each case which uses structure different from just stacking backbone, pooling, neck, and head or adds new components. This may be useful for models learned with AdversarialPropogation, Adversarial training or some cases of multiheaded learning, ensemble models or composite constructs.