src/transformers/models/auto/configuration_auto.py/_LazyConfigMapping/register
class _LazyConfigMapping:    def register(self, key, value):
        """
        Register a new configuration in this mapping.
        """
        if key in self._mapping.keys():
            raise ValueError(f"'{key}' is already used by a Transformers config, pick another name.")
        self._extra_content[key] = value