class_docstr: Utility class for calculating gradients.
class_docstr_tok: ["Utility","class","for","calculating","gradients","."]
code: def _sig2_grad(x, y, z, dz, op_dict, be):
      return (dz * z * (1.0 - z) * be.log(2.0), None)
  
imports: from __future__ import division
  from builtins import object, zip
  from neon.backends.backend import OpTreeNode, Tensor
  import numpy
  from functools import wraps
label_desc: Use image node_id 21 for calling the GradUtil obj's underlying member method code with example usage: obj._sig2_grad(x, y, z, dz, op_dict, be) and returns: unknown,  None