class_docstr: Elementwise inverse hyperbolic tangent function.
code: def backward(self, indexes, grad_outputs):
      (x,) = self.get_retained_inputs()
      (gy,) = grad_outputs
      gx = 1.0 / (1 - x**2) * gy
      return (gx,)
  
file_path: chainerrl/chainerrl/functions/arctanh.py
imports: from chainer.backends import cuda
  from chainer import function_node
  from chainer import utils
  from chainer.utils import type_check
label_desc: Use image node_id 3 for calling the Arctanh obj's underlying member method code with example usage: obj.backward(indexes, grad_outputs) and returns: gx