code: def ref_log_sigmoid(x):
      return np.log(1 / (1 + np.exp(-x, dtype=x.dtype)))
  
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_log_sigmoid(x) and returns: np