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

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

tests/models/upernet/test_modeling_upernet.py

@@ -105,6 +105,7 @@ class UperNetModelTester:
     def get_config(self):
         return UperNetConfig(
             backbone_config=self.get_backbone_config(),
+            backbone=None,
             hidden_size=64,
             pool_scales=[1, 2, 3, 6],
             use_auxiliary_head=True,