Returns the total period between when the first and last steps
        where logged. This usually correspnods to the total training time
        if there were no gaps in the training.

def get_total_time(self):
        """Returns the total period between when the first and last steps
        where logged. This usually correspnods to the total training time
        if there were no gaps in the training.
        """
        first_step = self.steps[0]
        last_step = self.steps[-1]
        seconds = self.history[last_step]["__timestamp__"] \
                  - self.history[first_step]["__timestamp__"]
        return datetime.timedelta(seconds=seconds)