code: def build_all_reduce_device_prefixes(job_name, num_tasks):
      
      if job_name != "localhost":
          return ["/job:%s/task:%d" % (job_name, d) for d in range(0, num_tasks)]
      else:
          assert num_tasks == 1
          return ["/job:%s" % job_name]
docs: Build list of device prefix names for all_reduce.
  
    Args:
      job_name: "worker", "ps" or "localhost".
      num_tasks: number of jobs across which device names should be generated.
  
    Returns:
       A list of device name prefix strings. Each element spells out the full
       host name without adding the device.
       e.g. "/job:worker/task:0"