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

tests/models/reformer/test_modeling_reformer.py

@@ -682,6 +682,10 @@ class ReformerLocalAttnModelTest(ReformerTesterMixin, GenerationTesterMixin, Mod
                 [expected_shape] * len(iter_hidden_states),
             )
 
+    @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 ReformerLSHAttnModelTest(
@@ -839,6 +843,10 @@ class ReformerLSHAttnModelTest(
     def test_past_key_values_format(self):
         pass
 
+    @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
 @require_sentencepiece