code: def _acc_grads(*lists_of_grads):
      """Accumulates lists of gradients."""
      acc_grads = []
      for grads in zip(*lists_of_grads):
          grads = [g for g in grads if g is not None]
          if grads:
              acc_grads.append(math_ops.add_n(grads))
          else:
              acc_grads.append(None)
      return acc_grads
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import functools
  import re
  import numpy
  import six
  from six.moves import xrange
  from ..None import contrib_framework
  from tensorflow.python.eager import backprop
  from tensorflow.python.framework import dtypes
  from tensorflow.python.framework import ops
  from tensorflow.python.layers import base
  from tensorflow.python.ops import array_ops
  from tensorflow.python.ops import control_flow_ops
  from tensorflow.python.ops import control_flow_util
  from tensorflow.python.ops import custom_gradient
  from tensorflow.python.ops import gradients_impl
  from tensorflow.python.ops import math_ops
  from tensorflow.python.ops import variable_scope
  from tensorflow.python.ops import variables
  from tensorflow.python.platform import tf_logging
  from tensorflow.python.util import nest
  from tensorflow.python.util import tf_inspect
label_desc: Use image node_id 1 for calling a global function with example usage: _acc_grads() and returns: acc_grads