Schedule with `cyclic=False` does not set `_schedule_time`

Description of issue: The schedule of the 3-point Winograd DFT example cannot be shown when cyclic=False. Error:

In [13]: schedule
Traceback (most recent call last):

  File /local/data1/miniconda3/lib/python3.8/site-packages/IPython/core/formatters.py:343 in __call__
    return method()

  File ~/git/B-ASIC/b_asic/schedule.py:359 in _repr_svg_
    self._plot_schedule()

  File ~/git/B-ASIC/b_asic/schedule.py:347 in _plot_schedule
    plt.axis([-1, self._schedule_time+1, 0, ypos])

AttributeError: 'Schedule' object has no attribute '_schedule_time'