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/tvp/test_modeling_tvp.py

@@ -124,6 +124,7 @@ class TVPModelTester:
         )
         return TvpConfig(
             backbone_config=resnet_config,
+            backbone=None,
             alpha=self.alpha,
             beta=self.beta,
             visual_prompter_type=self.visual_prompter_type,