[DINOv2] Add backbone class (#25520)
* First draft
* More improvements
* Fix all tests
* More improvements
* Add backbone test
* Improve docstring
* Address comments
* Rename attribute
* Remove expected output
* Update src/transformers/models/dinov2/modeling_dinov2.py
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>
* Fix style
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Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>
