[NAT, DiNAT] Add backbone class (#20654)

* Add first draft

* Add out_features attribute to config

* Add corresponding test

* Add Dinat backbone

* Add BackboneMixin

* Add Backbone mixin, improve tests

* Fix embeddings

* Fix bug

* Improve backbones

* Fix Nat backbone tests

* Fix Dinat backbone tests

* Apply suggestions

Co-authored-by: Niels Rogge <nielsrogge@Nielss-MacBook-Pro.local>

utils/check_repo.py

@@ -677,6 +677,8 @@ SHOULD_HAVE_THEIR_OWN_PAGE = [
     "MaskFormerSwinBackbone",
     "ResNetBackbone",
     "AutoBackbone",
+    "DinatBackbone",
+    "NatBackbone",
     "MaskFormerSwinConfig",
     "MaskFormerSwinModel",
 ]