code: def _flush(self, close=False):
          
          for channel in self.forward_channels:
              if close is True:
                  channel.queue.put_next(None)
              channel.queue._flush_writes()
          for channels in self.shuffle_channels:
              for channel in channels:
                  if close is True:
                      channel.queue.put_next(None)
                  channel.queue._flush_writes()
          for channels in self.shuffle_key_channels:
              for channel in channels:
                  if close is True:
                      channel.queue.put_next(None)
                  channel.queue._flush_writes()
          for channels in self.round_robin_channels:
              for channel in channels:
                  if close is True:
                      channel.queue.put_next(None)
                  channel.queue._flush_writes()
docs: Flushes remaining output records in the output queues to plasma.
  
          None is used as special type of record that is propagated from sources
          to sink to notify that the end of data in a stream.
  
          Attributes:
               close (bool): A flag denoting whether the channel should be
               also marked as 'closed' (True) or not (False) after flushing.