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/dpt/test_modeling_dpt_auto_backbone.py

@@ -95,6 +95,7 @@ class DPTModelTester:
     def get_config(self):
         return DPTConfig(
             backbone_config=self.get_backbone_config(),
+            backbone=None,
             neck_hidden_sizes=self.neck_hidden_sizes,
             fusion_hidden_size=self.fusion_hidden_size,
         )

tests/models/dpt/test_modeling_dpt_hybrid.py

@@ -130,6 +130,7 @@ class DPTModelTester:
             initializer_range=self.initializer_range,
             is_hybrid=self.is_hybrid,
             backbone_config=backbone_config,
+            backbone=None,
             backbone_featmap_shape=self.backbone_featmap_shape,
             neck_hidden_sizes=self.neck_hidden_sizes,
         )