code: def ref_relu(x):
      return np.maximum(x, 0)
  
file_path: nnabla/python/test/function/test_relu.py
imports: import pytest
  import numpy
  import nnabla
  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_relu(x) and returns: np