code: def backward(self, indexes, grad_outputs):
      (y,) = self.get_retained_outputs()
      (gy,) = grad_outputs
      return (gy * (y**3) * -0.5,)
  
imports: import numpy
  from chainer import backend
  from chainer.backends import cuda
  from chainer import function_node
  from chainer import utils
  from chainer.utils import type_check
  import chainerx
label_desc: Use image node_id 4 for calling the RsqrtGPU obj's underlying member method code with example usage: obj.backward(indexes, grad_outputs) and returns: unknown