code: def _initialize_transfer_threads(self):
          # type: (Downloader) -> None
          
          logger.debug('spawning {} transfer threads'.format(
              self._general_options.concurrency.transfer_threads))
          for _ in range(self._general_options.concurrency.transfer_threads):
              thr = threading.Thread(target=self._worker_thread_transfer)
              self._transfer_threads.append(thr)
              thr.start()
docs: Initialize transfer threads
          :param Downloader self: this