consistent ignore keys + make private (#8737)

* consistent ignore keys + make private

* style

* - authorized_missing_keys    => _keys_to_ignore_on_load_missing
  - authorized_unexpected_keys => _keys_to_ignore_on_load_unexpected

* move public doc of private attributes to private comment

tests/test_modeling_common.py

@@ -135,17 +135,17 @@ class ModelTesterMixin:
                 max_diff = np.amax(np.abs(out_1 - out_2))
                 self.assertLessEqual(max_diff, 1e-5)
 
-    def test_save_load_keys_to_never_save(self):
+    def test_save_load__keys_to_ignore_on_save(self):
         config, inputs_dict = self.model_tester.prepare_config_and_inputs_for_common()
 
         for model_class in self.all_model_classes:
             model = model_class(config)
-            keys_to_never_save = getattr(model, "keys_to_never_save", None)
-            if keys_to_never_save is None:
+            _keys_to_ignore_on_save = getattr(model, "_keys_to_ignore_on_save", None)
+            if _keys_to_ignore_on_save is None:
                 continue
 
             # check the keys are in the original state_dict
-            for k in keys_to_never_save:
+            for k in _keys_to_ignore_on_save:
                 self.assertIn(k, model.state_dict())
 
             # check that certain keys didn't get saved with the model
@@ -153,7 +153,7 @@ class ModelTesterMixin:
                 model.save_pretrained(tmpdirname)
                 output_model_file = os.path.join(tmpdirname, WEIGHTS_NAME)
                 state_dict_saved = torch.load(output_model_file)
-                for k in keys_to_never_save:
+                for k in _keys_to_ignore_on_save:
                     self.assertNotIn(k, state_dict_saved)
 
     def test_initialization(self):