message: Change zip(...)[1] to list(zip(...))[1], for python 3 compatibility.
diff: @@ -186,7 +186,7 @@ def _aggregate_grads(gradients):
  ret.append(g_list[0])
  else:
  # TODO(xpan): Aggregate IndexedSlices.
  - ret.append((g_list[0][0], math_ops.add_n(zip(*g_list)[1])))
  + ret.append((g_list[0][0], math_ops.add_n(list(zip(*g_list))[1])))
  return ret
  