code: def ref_function(*inputs, **params):
      y = 1
      for i in range(len(inputs)):
          y *= inputs[i]
      return y
  
file_path: nnabla/python/test/function/test_mul_n.py
imports: import pytest
  import numpy
  import nnabla
  import nnabla.functions
  from nbla_test_utils import list_context, function_tester
label_desc: Use image node_id 1 for calling a global function with example usage: ref_function() and returns: y