[DETR] Add num_channels attribute (#18714)

* Add num_channels attribute

* Fix code quality

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

tests/models/detr/test_modeling_detr.py

@@ -416,6 +416,26 @@ class DetrModelTest(ModelTesterMixin, GenerationTesterMixin, unittest.TestCase):
 
             self.assertTrue(outputs)
 
+    def test_greyscale_images(self):
+        config, inputs_dict = self.model_tester.prepare_config_and_inputs_for_common()
+
+        # use greyscale pixel values
+        inputs_dict["pixel_values"] = floats_tensor(
+            [self.model_tester.batch_size, 1, self.model_tester.min_size, self.model_tester.max_size]
+        )
+
+        # let's set num_channels to 1
+        config.num_channels = 1
+
+        for model_class in self.all_model_classes:
+            model = model_class(config)
+            model.to(torch_device)
+            model.eval()
+            with torch.no_grad():
+                outputs = model(**self._prepare_for_class(inputs_dict, model_class))
+
+            self.assertTrue(outputs)
+
     def test_initialization(self):
         config, inputs_dict = self.model_tester.prepare_config_and_inputs_for_common()