code: def _erf(self, dev=cpu_dev):
      np1 = np.random.random((2, 3)).astype(np.float32)
  
      x1 = tensor.from_numpy(np1)
      x1.to_device(dev)
      y1 = tensor.from_raw_tensor(singa_api.Erf(x1.data))
  
imports: from __future__ import division
  import unittest
  import math
  import numpy
  from singa import singa_wrap
  from singa import tensor
  from cuda_helper import gpu_dev, cpu_dev
label_desc: Use image node_id 11 for calling the TestAPI obj's underlying member method code with example usage: obj._erf(dev) without return types