[`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