class_docstr: Sign activation function.
  Computes the function f(x) = Sign(x).
  Uses straight-through estimator for bprop.
code: def __call__(self, x):
      self.inputs = self.be.array(x.get())
      return self.be.binarize(x, x, stochastic=False)
  
imports: from __future__ import division
  from neon.transforms.transform import Transform
label_desc: Use image node_id 2 for calling the Sign obj's underlying member method code with example usage: obj.__call__(x) and returns: self