class_docstr: Parametric Rectified Linear Unit function.
code: def backward(self, indexes, grad_outputs):
      x, W = self.get_retained_inputs()
      (gy,) = grad_outputs
      return PReLUFunctionGrad(
          x.data, _get_reduce_axes(W, x), _get_extended_shape(W, x)
      ).apply((x, W, gy))
  
imports: import numpy
  import chainer
  from chainer.backends import cuda
  from chainer import function_node
  from chainer.utils import type_check
label_desc: Use image node_id 4 for calling the PReLUFunction obj's underlying member method code with example usage: obj.backward(indexes, grad_outputs) and returns: PReLUFunctionGrad