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/maskformer/test_modeling_maskformer.py

@@ -102,6 +102,7 @@ class MaskFormerModelTester:
                 hidden_size=32,
                 num_heads=[1, 1, 2, 2],
             ),
+            backbone=None,
             decoder_config=DetrConfig(
                 decoder_ffn_dim=64,
                 decoder_layers=self.num_hidden_layers,