Whether this trial is due for checkpointing.

def should_checkpoint(self):
        """Whether this trial is due for checkpointing."""
        result = self.last_result or {}

        if result.get(DONE) and self.checkpoint_at_end:
            return True

        if self.checkpoint_freq:
            return result.get(TRAINING_ITERATION,
                              0) % self.checkpoint_freq == 0
        else:
            return False