code: def ref_top_k_grad_bw(x, g, k, abs, base_axis, **kw):
      return ref_top_k_grad(k, abs, base_axis, g).flatten()
  
file_path: nnabla/python/test/function/test_top_k_grad.py
imports: import pytest
  import numpy
  import nnabla.functions
  from nbla_test_utils import list_context, function_tester
label_desc: Use image node_id 3 for calling a global function with example usage: ref_top_k_grad_bw(x, g, k, abs, base_axis) and returns: ref_top_k_grad