code: def _operation(self):
      c0 = self._compile_c0()
      x, xs = self._compile_x()
      loop_bounds, loop_main = self._compile_loop(xs)
      before, main, after = self._compile_procedure(xs)
      return "\n".join(
          c0 + x + loop_bounds + before + loop_main(main) + after
      )
  
imports: from six import moves
  from chainer.backends import cuda
  from chainer.utils import conv_nd_kernel
label_desc: Use image node_id 15 for calling the PoolingNDKernelBackward obj's underlying member method code with example usage: obj._operation() and returns: str