[Backbone] Use load_backbone instead of AutoBackbone.from_config (#28661)

* Enable instantiating model with pretrained backbone weights

* Remove doc updates until changes made in modeling code

* Use load_backbone instead

* Add use_timm_backbone to the model configs

* Add missing imports and arguments

* Update docstrings

* Make sure test is properly configured

* Include recent DPT updates

utils/check_config_attributes.py

@@ -219,7 +219,11 @@ def check_attribute_being_used(config_class, attributes, default_value, source_s
         "out_features",
         "out_indices",
         "sampling_rate",
+        # backbone related arguments passed to load_backbone
         "use_pretrained_backbone",
+        "backbone",
+        "backbone_config",
+        "use_timm_backbone",
     ]
     attributes_used_in_generation = ["encoder_no_repeat_ngram_size"]