[test] update test_past_key_values_format (#37614)

allow custom shapes

tests/models/mixtral/test_modeling_mixtral.py

@@ -291,10 +291,6 @@ class MixtralModelTest(ModelTesterMixin, GenerationTesterMixin, PipelineTesterMi
             (self.model_tester.batch_size, self.model_tester.seq_length, self.model_tester.num_labels),
         )
 
-    @unittest.skip(reason="Mixtral uses GQA on all models so the KV cache is a non standard format")
-    def test_past_key_values_format(self):
-        pass
-
     @require_flash_attn
     @require_torch_gpu
     @pytest.mark.flash_attn_test