Add TimmBackbone model (#22619)

* Add test_backbone for convnext

* Add TimmBackbone model

* Add check for backbone type

* Tidying up - config checks

* Update convnextv2

* Tidy up

* Fix indices & clearer comment

* Exceptions for config checks

* Correclty update config for tests

* Safer imports

* Safer safer imports

* Fix where decorators go

* Update import logic and backbone tests

* More import fixes

* Fixup

* Only import all_models if torch available

* Fix kwarg updates in from_pretrained & main rebase

* Tidy up

* Add tests for AutoBackbone

* Tidy up

* Fix import error

* Fix up

* Install nattan in doc_test_job

* Revert back to setting self._out_xxx directly

* Bug fix - out_indices mapping from out_features

* Fix tests

* Dont accept output_loading_info for Timm models

* Set out_xxx and don't remap

* Use smaller checkpoint for test

* Don't remap timm indices - check out_indices based on stage names

* Skip test as it's n/a

* Apply suggestions from code review

Co-authored-by: Sylvain Gugger <[email protected]>

* Cleaner imports / spelling is hard

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Co-authored-by: Sylvain Gugger <[email protected]>