code: def test_construct_vol_defaults(self):
      # Check the None, None
      dtype = np.float64
      vol_fn, _ = bm_utils.construct_vol_data(None, None, 2, dtype)
      times = tf.constant([0.5, 1.0, 2.0, 3.0], dtype=dtype)
      vols = self.evaluate(vol_fn(times))
      np.testing.assert_array_equal(vols.shape, [4, 2, 2])
  
imports: import numpy
  import tensorflow.compat.v2
  from tensorflow.python.framework import test_util
  from tf_quant_finance.models.legacy import brownian_motion_utils
label_desc: Use image node_id 6 for calling the BrownianMotionUtilsTest obj's underlying member method code with example usage: obj.test_construct_vol_defaults() without return types