code: def apply_default_prefetch(input_source_or_dataflow, trainer):
      
      if not isinstance(input_source_or_dataflow, InputSource):
          # to mimic same behavior of the old trainer interface
          if type(trainer) == SimpleTrainer:
              input = FeedInput(input_source_or_dataflow)
          else:
              logger.info("Automatically applying QueueInput on the DataFlow.")
              input = QueueInput(input_source_or_dataflow)
      else:
          input = input_source_or_dataflow
      if hasattr(trainer, 'devices'):
          towers = trainer.devices
          if len(towers) > 1:
              # seem to only improve on >1 GPUs
              assert not isinstance(trainer, SimpleTrainer)
  
              if isinstance(input, FeedfreeInput) and \
                 not isinstance(input, (StagingInput, DummyConstantInput)):
                  logger.info("Automatically applying StagingInput on the DataFlow.")
                  input = StagingInput(input)
      return input
docs: Apply a set of default rules to make a fast :class:`InputSource`.
  
      Args:
          input_source_or_dataflow(InputSource | DataFlow):
          trainer (Trainer):
  
      Returns:
          InputSource