def BackwardPropagate(self, step):
    """Backprop through the network.

    Args:
      step: Training step.
    """
    losses = []
    for node in reversed(self.node_list):
      loss = self.ComputeDown(node, step)
      if loss:
        losses.append(loss)
    return losses
Backprop through the network.

    Args:
      step: Training step.
    
deepnet/neuralnet.py