code: def test_channels(self):
      (
          config,
          _,
      ) = self.model_tester.prepare_config_and_inputs_for_common()
  
      for model_class in self.all_model_classes:
          model = model_class(config)
          self.assertEqual(
              len(model.channels), len(config.out_features)
          )
          num_features = model.num_features
          out_indices = [
              config.stage_names.index(feat)
              for feat in config.out_features
          ]
          out_channels = [num_features[idx] for idx in out_indices]
          self.assertListEqual(model.channels, out_channels)
  
          new_config = copy.deepcopy(config)
          new_config.out_features = None
          model = model_class(new_config)
          self.assertEqual(len(model.channels), 1)
          self.assertListEqual(model.channels, [num_features[-1]])
  
          new_config = copy.deepcopy(config)
          new_config.out_indices = None
          model = model_class(new_config)
          self.assertEqual(len(model.channels), 1)
          self.assertListEqual(model.channels, [num_features[-1]])
  
file_path: transformers/tests/test_backbone_common.py
imports: import copy
  import inspect
  import tempfile
  from transformers.testing_utils import require_torch, torch_device
  from transformers.utils.backbone_utils import BackboneType
label_desc: Use image node_id 4 for calling the BackboneTesterMixin obj's underlying member method code with example usage: obj.test_channels() without return types