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

tests/generation/test_utils.py

@@ -1597,9 +1597,7 @@ class GenerationTesterMixin:
                 attn_weights = out[attn_name] if attn_name == attention_names[0] else out[attn_name][-1]
                 self.assertEqual(sum([w.sum().item() for w in attn_weights]), 0.0)
 
-    # TODO (joao): this test is actually not slow :) However, it is not passing in some models (e.g. GPTNeoX) and the
-    # fix for some models is quite lengthy. Being slow means it doesn't block our push CI while we fix it.
-    @slow
+    @slow  # TODO (Joao): fix GPTBigCode
     def test_left_padding_compatibility(self):
         # The check done in this test is fairly difficult -- depending on the model architecture, passing the right
         # position index for the position embeddings can still result in a different output, due to numerical masking.