code: def _reduce(grad_list):
      if not grad_list:
          return None
      if len(grad_list) >= 2:
          grad_list[:] = [chainer.functions.add(*grad_list)]
      return grad_list[0]
  
imports: import os
  import shutil
  import sys
  import traceback
  import six
  import chainer
label_desc: Use image node_id 1 for calling a global function with example usage: _reduce(grad_list) and returns: grad_list, None