Remove redundant test_head_masking = True flags in test files (#9858)

* Remove redundant test_head_masking = True flags

* Remove all redundant test_head_masking flags in PyTorch test_modeling_* files

* Make test_head_masking = True as a default choice in test_modeling_tf_commong.py

* Remove all redundant test_head_masking flags in TensorFlow
test_modeling_tf_* files

* Put back test_head_masking=False fot TFT5 models

tests/test_modeling_tf_blenderbot_small.py

@@ -179,7 +179,6 @@ class TFBlenderbotSmallModelTest(TFModelTesterMixin, unittest.TestCase):
     all_generative_model_classes = (TFBlenderbotSmallForConditionalGeneration,) if is_tf_available() else ()
     is_encoder_decoder = True
     test_pruning = False
-    test_head_masking = True
 
     def setUp(self):
         self.model_tester = TFBlenderbotSmallModelTester(self)