[Tests] Improve flax test_attention_outputs (#21486)

improving flax tests

tests/test_modeling_flax_common.py

@@ -650,6 +650,9 @@ class FlaxModelTesterMixin:
             check_hidden_states_output(inputs_dict, config, model_class)
 
     def test_attention_outputs(self):
+        if not self.has_attentions:
+            self.skipTest(reason="Model does not output attentions")
+
         config, inputs_dict = self.model_tester.prepare_config_and_inputs_for_common()
         config.return_dict = True