[AutoBackbone] Add test (#26094)

* Add test

* Add config_class

tests/test_backbone_common.py

@@ -15,6 +15,7 @@
 
 import copy
 import inspect
+import tempfile
 
 from transformers.testing_utils import require_torch, torch_device
 from transformers.utils.backbone_utils import BackboneType
@@ -72,6 +73,16 @@ class BackboneTesterMixin:
             expected_arg_names = ["pixel_values"]
             self.assertListEqual(arg_names[:1], expected_arg_names)
 
+    def test_config_save_pretrained(self):
+        config_class = self.config_class
+        config_first = config_class(out_indices=[0, 1, 2, 3])
+
+        with tempfile.TemporaryDirectory() as tmpdirname:
+            config_first.save_pretrained(tmpdirname)
+            config_second = self.config_class.from_pretrained(tmpdirname)
+
+        self.assertEqual(config_second.to_dict(), config_first.to_dict())
+
     def test_channels(self):
         config, _ = self.model_tester.prepare_config_and_inputs_for_common()