Add ConvNeXT (#15277)

* First draft

* Add conversion script

* Improve conversion script

* Improve docs and implement tests

* Define model output class

* Fix tests

* Fix more tests

* Add model to README

* Apply suggestions from code review

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* Apply more suggestions from code review

* Apply suggestions from code review

* Rename dims to hidden_sizes

* Fix equivalence test

* Rename gamma to gamma_parameter

* Clean up conversion script

* Add ConvNextFeatureExtractor

* Add corresponding tests

* Implement feature extractor correctly

* Make implementation cleaner

* Add ConvNextStem class

* Improve design

* Update design to also include encoder

* Fix gamma parameter

* Use sample docstrings

* Finish conversion, add center cropping

* Replace nielsr by facebook, make feature extractor tests smaller

* Fix integration test

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

docs/source/_toctree.yml

@@ -150,6 +150,8 @@
       title: CamemBERT
     - local: model_doc/canine
       title: CANINE
+    - local: model_doc/convnext
+      title: ConvNeXT
     - local: model_doc/clip
       title: CLIP
     - local: model_doc/convbert