class_docstr: Sigmoid activation function.
class_docstr_tok: ["Sigmoid","activation","function","."]
code: def gradient(z):
      s = Sigmoid.apply(z)
      return s * (1 - s)
  
file_path: river/river/neural_net/activations.py
imports: from __future__ import annotations
  import abc
  import numpy
label_desc: Use image node_id 2 for calling the Sigmoid obj's underlying member method code with example usage: obj.gradient(z) and returns: unknown