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

tests/models/mbart/test_modeling_mbart.py

@@ -692,3 +692,7 @@ class MBartStandaloneDecoderModelTest(ModelTesterMixin, GenerationTesterMixin, u
     def test_retain_grad_hidden_states_attentions(self):
         # decoder cannot keep gradients
         return
+
+    @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