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.

src/transformers/testing_utils.py

@@ -58,6 +58,7 @@ from .utils import (
     is_ipex_available,
     is_jumanpp_available,
     is_librosa_available,
+    is_natten_available,
     is_onnx_available,
     is_pandas_available,
     is_phonemizer_available,
@@ -282,6 +283,16 @@ def require_timm(test_case):
     return unittest.skipUnless(is_timm_available(), "test requires Timm")(test_case)
 
 
+def require_natten(test_case):
+    """
+    Decorator marking a test that requires NATTEN.
+
+    These tests are skipped when NATTEN isn't installed.
+
+    """
+    return unittest.skipUnless(is_natten_available(), "test requires natten")(test_case)
+
+
 def require_torch(test_case):
     """
     Decorator marking a test that requires PyTorch.