code: def _piecewise_learning_rate(step, boundaries, values):
    
    values = [1.0] + values
    boundaries = [float(x) for x in boundaries]
    return tf.train.piecewise_constant(
        step, boundaries, values, name="piecewise_lr")
docs: Scale learning rate according to the given schedule.
  
    Multipliers are not cumulative.
  
    Args:
      step: global step
      boundaries: List of steps to transition on.
      values: Multiplier to apply at each boundary transition.
  
    Returns:
      Scaled value for the learning rate.