code: def reduce_func(self, nodes):
      h_tild = nodes.mailbox["h"].sum(axis=1)
      f = self.U_f(nodes.mailbox["h"]).sigmoid()
      c = (f * nodes.mailbox["c"]).sum(axis=1)
      return {"iou": self.U_iou(h_tild), "c": c}
  
imports: import itertools
  import time
  import dgl
  import mxnet
  import networkx
  import numpy
  from mxnet import gluon
label_desc: Use image node_id 2 for calling the ChildSumTreeLSTMCell obj's underlying member method code with example usage: obj.reduce_func(nodes) and returns: dict