code: def backward(self, indexes, grad_outputs):
      x0, x1 = self.get_retained_inputs()
      (gy,) = grad_outputs
      diff = x0 - x1
      delta = self.delta
  
      gx = chainer.functions.clip(diff, -delta, delta)
  
      if self.reduce == "sum_along_second_axis":
          gy = chainer.functions.expand_dims(gy, 1)
      gx = chainer.functions.broadcast_to(gy, gx.shape) * gx
      return gx, -gx
  
file_path: chainer/chainer/functions/loss/huber_loss.py
imports: import chainer
  from chainer import backend
  from chainer import function_node
  from chainer import utils
  from chainer.utils import type_check
label_desc: Use image node_id 4 for calling the HuberLoss obj's underlying member method code with example usage: obj.backward(indexes, grad_outputs) and returns: gx,  unknown