code: def Sigmoid():
      r"""Returns a layer that computes the sigmoid function.
  
      .. math::
          f(x) = \frac{1}{1 + e^{-x}}
      """
      return Fn("Sigmoid", lambda x: fastmath.expit(x))
  
imports: from trax import fastmath
  from trax.fastmath import numpy
  from trax.layers import base
  from trax.layers.assert_shape import assert_shape
  from trax.layers.base import Fn
label_desc: Use image node_id 8 for calling a global function with example usage: Sigmoid() and returns: Fn