message: hard_sigmoid doc
  The description wasn't detailed enough. Added a tiny bit more explanation and a reference.
  I don't know if this is the proper formatting of the url.
diff: @@ -434,6 +434,8 @@ def hard_sigmoid(x):
  """Hard sigmoid activation function.
  A faster approximation of the sigmoid activation.
  + Piecewise linear approximation of the sigmoid function.
  + Ref: 'https://paperswithcode.com/method/hard-sigmoid'
  For example:
  