Make ImageProcessorMixin compatible with subfolder kwarg (#21725)

* Add subfolder support

* Add kwarg docstring

* formatting fix

* Add test

tests/test_image_processing_common.py

@@ -311,3 +311,14 @@ class ImageProcessorPushToHubTester(unittest.TestCase):
         )
         # Can't make an isinstance check because the new_image_processor is from the CustomImageProcessor class of a dynamic module
         self.assertEqual(new_image_processor.__class__.__name__, "CustomImageProcessor")
+
+    def test_image_processor_from_pretrained_subfolder(self):
+        with self.assertRaises(OSError):
+            # config is in subfolder, the following should not work without specifying the subfolder
+            _ = AutoImageProcessor.from_pretrained("hf-internal-testing/stable-diffusion-all-variants")
+
+        config = AutoImageProcessor.from_pretrained(
+            "hf-internal-testing/stable-diffusion-all-variants", subfolder="feature_extractor"
+        )
+
+        self.assertIsNotNone(config)