def modify_model_backprop(model, backprop_modifier):
raise NotImplementedError('<STR_LIT>')<EOL>
Creates a copy of model by modifying all activations to use a custom op to modify the backprop behavior.

   Args:
       model:  The `keras.models.Model` instance.
       backprop_modifier: One of `{'guided', 'rectified'}`

   Returns:
       A copy of model with modified activations for backwards pass.