code: def setup(job, input_file_id, n, down_checkpoints):
      
      # Write the input file to the file store
      job.fileStore.logToMaster("Starting the merge sort")
      return job.addChildJobFn(down,
                               input_file_id, n,
                               down_checkpoints=down_checkpoints,
                               memory='600M').rv()
docs: Sets up the sort.
      Returns the FileID of the sorted file