Bypass the require a config warning for marge layers
Bypass the require a config warning for marge layers

Bypass the warning `warnings.warn('Custom mask layers require a config and must override '` when saving a model containing marge layers like `Add` / `Multiply`.
- Before:
  ```py
  inputs = keras.layers.Input(shape=(32, 32, 3))
  keras.models.Model(inputs, keras.layers.Add()([inputs, inputs])).save('aa.h5')
  # WARNING:tensorflow:Compiled the loaded model, but the compiled metrics have yet to be built. `model.compile_metrics` will be empty until you train or evaluate the model.
  # /opt/anaconda3/lib/python3.8/site-packages/keras/utils/generic_utils.py:494: CustomMaskWarning: Custom mask layers require a config and must override get_config. When loading, the custom mask layer must be passed to the custom_objects argument.
  #   warnings.warn('Custom mask layers require a config and must override '
  ```
- After:
  ```py
  inputs = keras.layers.Input(shape=(32, 32, 3))
  keras.models.Model(inputs, keras.layers.Add()([inputs, inputs])).save('aa.h5')
  # WARNING:tensorflow:Compiled the loaded model, but the compiled metrics have yet to be built. `model.compile_metrics` will be empty until you train or evaluate the model.
  ```
keras-team/keras,keras-team/keras