code: def rfunc(nodes):
      msg = F.sum(nodes.mailbox["m"], 1)
      return {"x": nodes.data["x"] + msg}
  
file_path: dgl/tests/python/common/test_propagate.py
imports: import unittest
  import backend
  import dgl
  import networkx
  from utils import check_fail, parametrize_idtype
label_desc: Use image node_id 3 for calling a global function with example usage: rfunc(nodes) and returns: dict