Fix head_mask for model templates

templates/adding_a_new_model/cookiecutter-template-{{cookiecutter.modelname}}/test_modeling_tf_{{cookiecutter.lowercase_modelname}}.py

@@ -252,6 +252,8 @@ class TF{{cookiecutter.camelcase_modelname}}ModelTest(TFModelTesterMixin, unitte
         else ()
     )
 
+    test_head_masking = False
+
     def setUp(self):
         self.model_tester = TF{{cookiecutter.camelcase_modelname}}ModelTester(self)
         self.config_tester = ConfigTester(self, config_class={{cookiecutter.camelcase_modelname}}Config, hidden_size=37)
@@ -475,6 +477,7 @@ class TF{{cookiecutter.camelcase_modelname}}ModelTest(TFModelTesterMixin, unitte
     all_generative_model_classes = (TF{{cookiecutter.camelcase_modelname}}ForConditionalGeneration,) if is_tf_available() else ()
     is_encoder_decoder = True
     test_pruning = False
+    test_head_masking = False
 
     def setUp(self):
         self.model_tester = TF{{cookiecutter.camelcase_modelname}}ModelTester(self)