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

tests/models/prophetnet/test_modeling_prophetnet.py

@@ -1146,6 +1146,10 @@ class ProphetNetStandaloneDecoderModelTest(ModelTesterMixin, GenerationTesterMix
         # 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
+
 
 @require_torch
 class ProphetNetStandaloneEncoderModelTest(ModelTesterMixin, unittest.TestCase):