code: def reduce_func(self, nodes):
      # concatenate h_jl for equation (1), (2), (3), (4)
      h_cat = nodes.mailbox["h"].view(nodes.mailbox["h"].size(0), -1)
      # equation (2)
      f = th.sigmoid(self.U_f(h_cat)).view(*nodes.mailbox["h"].size())
      # second term of equation (5)
      c = th.sum(f * nodes.mailbox["c"], 1)
      return {"iou": self.U_iou(h_cat), "c": c}
  
imports: import os
  from collections import namedtuple
  import dgl
  from dgl.data.tree import SSTDataset
  import matplotlib.pyplot
  import networkx
  import torch
  import torch.nn
  import dgl.function
  import torch
  import torch.nn.functional
  from torch.utils.data import DataLoader
label_desc: Use image node_id 3 for calling the TreeLSTMCell obj's underlying member method code with example usage: obj.reduce_func(nodes) and returns: dict