Add BeitBackbone (#25952)
* First draft
* Add backwards compatibility
* More improvements
* More improvements
* Improve error message
* Address comment
* Add conversion script
* Fix style
* Update code snippet
* Adddress comment
* Apply suggestions from code review
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>
---------
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>
