[Backbone] Use load_backbone instead of AutoBackbone.from_config (#28661)

* Enable instantiating model with pretrained backbone weights

* Remove doc updates until changes made in modeling code

* Use load_backbone instead

* Add use_timm_backbone to the model configs

* Add missing imports and arguments

* Update docstrings

* Make sure test is properly configured

* Include recent DPT updates

tests/models/conditional_detr/test_modeling_conditional_detr.py

@@ -443,6 +443,7 @@ class ConditionalDetrModelTest(ModelTesterMixin, GenerationTesterMixin, Pipeline
 
         # let's pick a random timm backbone
         config.backbone = "tf_mobilenetv3_small_075"
+        config.use_timm_backbone = True
 
         for model_class in self.all_model_classes:
             model = model_class(config)