code: def ref_hard_tanh_backward(x, dy, **kw):
      return np.array([dy if -1 <= i <= 1 else 0 for i in np.nditer(x)])
  
file_path: nnabla/python/test/function/test_hard_tanh.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_hard_tanh_backward(x, dy) and returns: np