Add Neighborhood Attention Transformer (NAT) and Dilated NAT (DiNAT) models (#20219)

* Add DiNAT

* Adds DiNAT + tests

* Minor fixes

* Added HF model

* Add natten to dependencies.

* Cleanup

* Minor fixup

* Reformat

* Optional NATTEN import.

* Reformat & add doc to _toctree

* Reformat (finally)

* Dummy objects for DiNAT

* Add NAT + minor changes

Adds NAT as its own independent model + docs, tests
Adds NATTEN to ext deps to ensure ci picks it up.

* Remove natten from `all` and `dev-torch` deps, add manual pip install to ci tests

* Minor fixes.

* Fix READMEs.

* Requested changes to docs + minor fixes.

* Requested changes.

* Add NAT/DiNAT tests to layoutlm_job

* Correction to Dinat doc.

* Requested changes.

utils/documentation_tests.txt

@@ -66,6 +66,8 @@ src/transformers/models/deit/modeling_deit.py
 src/transformers/models/deit/modeling_tf_deit.py
 src/transformers/models/detr/configuration_detr.py
 src/transformers/models/detr/modeling_detr.py
+src/transformers/models/dinat/configuration_dinat.py
+src/transformers/models/dinat/modeling_dinat.py
 src/transformers/models/distilbert/configuration_distilbert.py
 src/transformers/models/dpr/configuration_dpr.py
 src/transformers/models/dpt/modeling_dpt.py
@@ -113,6 +115,8 @@ src/transformers/models/mobilebert/modeling_tf_mobilebert.py
 src/transformers/models/mobilenet_v2/modeling_mobilenet_v2.py
 src/transformers/models/mobilevit/modeling_mobilevit.py
 src/transformers/models/mobilevit/modeling_tf_mobilevit.py
+src/transformers/models/nat/configuration_nat.py
+src/transformers/models/nat/modeling_nat.py
 src/transformers/models/nezha/configuration_nezha.py
 src/transformers/models/openai/configuration_openai.py
 src/transformers/models/opt/configuration_opt.py