code: def add_extra_model_fields(sender, **kwargs):
      
      model_key = sender._meta.app_label, sender._meta.model_name
      for field_name, field in fields.get(model_key, {}):
          field.contribute_to_class(sender, field_name)
docs: Injects custom fields onto the given sender model as defined
      by the ``EXTRA_MODEL_FIELDS`` setting. This is a closure over
      the "fields" variable.