<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def fetch_model(self, source: str, file: Union[str, BinaryIO], chunk_size: int=DEFAULT_DOWNLOAD_CHUNK_SIZE) -> None: """Download the model from GCS."""

        download_http(source, file, self._log, chunk_size)