code: def ref_grad_prune(x, dy, rate, **kw):
      # pass through gradient from output to input
      return dy.flatten()
  
file_path: nnabla/python/test/function/test_prune.py
imports: import pytest
  import numpy
  import nnabla.functions
  from nbla_test_utils import list_context
label_desc: Use image node_id 2 for calling a global function with example usage: ref_grad_prune(x, dy, rate) and returns: dy