Generate: skip left-padding tests on old models (#23437)

tests/models/ctrl/test_modeling_ctrl.py

@@ -249,6 +249,10 @@ class CTRLModelTest(ModelTesterMixin, GenerationTesterMixin, PipelineTesterMixin
             model = CTRLModel.from_pretrained(model_name)
             self.assertIsNotNone(model)
 
+    @unittest.skip("The model doesn't support left padding")  # and it's not used enough to be worth fixing :)
+    def test_left_padding_compatibility(self):
+        pass
+
 
 @require_torch
 class CTRLModelLanguageGenerationTest(unittest.TestCase):