Backbone kwargs in config (#28784)
* Enable instantiating model with pretrained backbone weights
* Clarify pretrained import
* Use load_backbone instead
* Add backbone_kwargs to config
* Pass kwargs to constructors
* Fix up
* Input verification
* Add tests
* Tidy up
* Update tests/utils/test_backbone_utils.py
Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>
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Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>
