code: def mean_udf(nodes):
      return {"h_N": nodes.mailbox["m"].mean(1)}
  
imports: import os
  import dgl
  import dgl.function
  import torch
  import torch.nn
  import torch.nn.functional
  import dgl.data
label_desc: Use image node_id 3 for calling a global function with example usage: mean_udf(nodes) and returns: dict