code: def test_mean_all_float64_dtype(self, xp, dtype):
      a = xp.full((2, 3, 4), 123456789, dtype=dtype)
      return xp.mean(a, dtype=numpy.float64)
  
imports: import math
  import numpy
  import pytest
  import cupy
  from cupy import testing
label_desc: Use image node_id 5 for calling the TestMeanVar obj's underlying member method code with example usage: obj.test_mean_all_float64_dtype(xp, dtype) and returns: xp