class_docstr: Sign activation function.
  Computes the function f(x) = Sign(x).
  Uses straight-through estimator for bprop.
code: def bprop(self, x):
      return self.be.less_equal(self.be.absolute(self.inputs), 1)
  
imports: from __future__ import division
  from neon.transforms.transform import Transform
label_desc: Use image node_id 3 for calling the Sign obj's underlying member method code with example usage: obj.bprop(x) and returns: self