code: def test_nanmean_all_nan(self, xp):
      a = xp.zeros((3, 4))
      a[:] = xp.nan
      return xp.nanmean(a)
  
imports: import math
  import numpy
  import pytest
  import cupy
  from cupy import testing
label_desc: Use image node_id 4 for calling the TestNanMeanAdditional obj's underlying member method code with example usage: obj.test_nanmean_all_nan(xp) and returns: xp