Tag tests as slow ⌛ (#21537)

begone slow tests

tests/models/mobilevit/test_modeling_tf_mobilevit.py

@@ -268,6 +268,7 @@ class MobileViTModelTest(TFModelTesterMixin, unittest.TestCase):
         not is_tf_available() or len(tf.config.list_physical_devices("GPU")) == 0,
         reason="TF does not support backprop for grouped convolutions on CPU.",
     )
+    @slow
     def test_keras_fit(self):
         config, _ = self.model_tester.prepare_config_and_inputs_for_common()