src/transformers/models/auto/configuration_auto.py/_LazyConfigMapping/__getitem__
class _LazyConfigMapping:    def __getitem__(self, key):
        if key in self._extra_content:
            return self._extra_content[key]
        if key not in self._mapping:
            raise KeyError(key)
        value = self._mapping[key]
        module_name = model_type_to_module_name(key)
        if module_name not in self._modules:
            self._modules[module_name] = importlib.import_module(f".{module_name}", "transformers.models")
        if hasattr(self._modules[module_name], value):
            return getattr(self._modules[module_name], value)

        # Some of the mappings have entries model_type -> config of another model type. In that case we try to grab the
        # object at the top level.
        transformers_module = importlib.import_module("transformers")
        return getattr(transformers_module, value)