code: def ref_crelu(x, axis):
      return np.concatenate(
          [np.maximum(x, 0), np.maximum(-x, 0)], axis=axis
      )
  
imports: import pytest
  import numpy
  import nnabla.functions
  from nbla_test_utils import list_context
label_desc: Use image node_id 1 for calling a global function with example usage: ref_crelu(x, axis) and returns: np