Enable instantiating model with pretrained backbone weights (#28214)

* Enable instantiating model with pretrained backbone weights

* Update tests so backbone checkpoint isn't passed in

* Remove doc updates until changes made in modeling code

* Clarify pretrained import

* Update configs - docs and validation check

* Update src/transformers/utils/backbone_utils.py

Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

* Clarify exception message

* Update config init in tests

* Add test for when use_timm_backbone=True

* Small test updates

---------

Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

tests/models/deformable_detr/test_modeling_deformable_detr.py

@@ -149,7 +149,9 @@ class DeformableDetrModelTester:
             encoder_n_points=self.encoder_n_points,
             decoder_n_points=self.decoder_n_points,
             use_timm_backbone=False,
+            backbone=None,
             backbone_config=resnet_config,
+            use_pretrained_backbone=False,
         )
 
     def prepare_config_and_inputs_for_common(self):
@@ -518,6 +520,8 @@ class DeformableDetrModelTest(ModelTesterMixin, GenerationTesterMixin, PipelineT
 
         # let's pick a random timm backbone
         config.backbone = "tf_mobilenetv3_small_075"
+        config.use_timm_backbone = True
+        config.backbone_config = None
 
         for model_class in self.all_model_classes:
             model = model_class(config)