code: def backward(self, inputs, grads):
      xp = backend.get_array_module(*inputs)
      stack, indices = inputs
      g, gs = grads
      if gs is None:
          gs = xp.zeros_like(stack)
      if g is not None:
          gs[xp.arange(len(indices)), indices] += g
      return gs, None
  
file_path: chainer/examples/sentiment/thin_stack.py
imports: import chainer
  from chainer import backend
  from chainer.utils import type_check
label_desc: Use image node_id 3 for calling the ThinStackGet obj's underlying member method code with example usage: obj.backward(inputs, grads) and returns: gs,  None