'Get left and right gradient increments from back-propagation.
Arguments:
grad_node (GradNode): The GradNode to perform gradient
back-propagation on.'

@staticmethod
def get_grad_back(grad_node):

    if (not grad_node):
        return None
    x = (grad_node.left.op_tree if grad_node.left else None)
    y = (grad_node.right.op_tree if grad_node.right else None)
    z = grad_node.op_tree
    dz = grad_node.grad_op_tree
    op_dict = z[0]
    be = grad_node.ad.be
    op = grad_node.op_tree[0]['op']
    grad_increments = grad_map[op](x, y, z, dz, op_dict, be)
    left_increment = GradUtil._unbroadcast(grad_increments[0], x, be)
    right_increment = GradUtil._unbroadcast(grad_increments[1], y, be)
    return (left_increment, right_increment)
