[WIP] Make Backbone parent classes to allow handling all subclass presets and support extensions

[smitlg]

What does this PR do?

This PR makes supports easy tree extensions and allows access to all subclass presets to a parent class.

Who can review?

cc @divyashreepathihalli

[smitlg]

@divyashreepathihalli

I found one local failure:

FAILED keras_cv/models/object_detection/retinanet/retinanet_test.py::RetinaNetTest::test_tf_dataset_data_generator - ValueError: Preset has type `ResNetBackbone` which is not a a subclass of calling class `ResNet50Backbone`...

Debugging into this, I see that kaggle config json has registered_name: "keras_cv>ResNetBackbone". To make this PR work, we need to update the config to have ResNet50Backbone.

[divyashreepathihalli]

@divyashreepathihalli

I found one local failure:

FAILED keras_cv/models/object_detection/retinanet/retinanet_test.py::RetinaNetTest::test_tf_dataset_data_generator - ValueError: Preset has type `ResNetBackbone` which is not a a subclass of calling class `ResNet50Backbone`...

Debugging into this, I see that kaggle config json has registered_name: "keras_cv>ResNetBackbone". To make this PR work, we need to update the config to have ResNet50Backbone.

What you see on kaggle is the standard format that is followed for all models in KerasCV and KerasNLP. That will not be changed.