[test] update test_past_key_values_format (#37614)

allow custom shapes

tests/models/mllama/test_modeling_mllama.py

@@ -409,7 +409,7 @@ class MllamaForConditionalGenerationModelTest(ModelTesterMixin, GenerationTester
         pass
 
     @pytest.mark.generate
-    # overridden because mllama has special cache for self and cross attentions
+    # overridden because mllama is not an encoder-decoder model, but has encoder-decoder-like cache
     def test_past_key_values_format(self):
         # Test that the KV cache is formatted correctly. Exceptions need to explicitly overwrite this test. Having a
         # standard KV cache format is important for a consistent API (and for advanced generation methods).