code: def ref_celu(x, alpha, axis):
      elu1 = np.maximum(0.0, x) + alpha * (np.exp(np.minimum(0, x)) - 1)
      elu2 = np.maximum(0.0, -x) + alpha * (
          np.exp(np.minimum(0, -x)) - 1
      )
      return np.concatenate([elu1, elu2], 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_celu(x, alpha, axis) and returns: np