code: def ref_norm(x, p, axis, keepdims):
      if p is None:
          p = 2.0
      x = np.abs(x)
      x = np.power(x, p)
      x = np.sum(x, axis, keepdims=keepdims)
      x = np.power(x, 1.0 / p)
      return x
  
imports: import pytest
  import numpy
  import nnabla.functions
  from nbla_test_utils import function_tester, list_context
label_desc: Use image node_id 1 for calling a global function with example usage: ref_norm(x, p, axis, keepdims) and returns: x