docstr: Tests that the `time_step` should be supplied if Euler scheme is used.
docstr_tok: ["Tests","that","the","`","time_step","`","should","be","supplied","if","Euler","scheme","is","used","."]
code: def test_time_step_not_supplied(self):
      """Tests that the `time_step` should be supplied if Euler scheme is used."""
      dtype = tf.float64
  
      def volatility_fn(t):
          del t
          return self.volatility
  
      process = tff.models.hull_white.VectorHullWhiteModel(
          dim=2,
          mean_reversion=self.mean_reversion,
          volatility=volatility_fn,
          initial_discount_rate_fn=self.instant_forward_rate_2d_fn,
          dtype=dtype,
      )
      with self.assertRaises(ValueError):
          process.sample_paths([0.1, 2.0], num_samples=100)
  
imports: from absl.testing import parameterized
  import numpy
  import tensorflow.compat.v2
  import tf_quant_finance
  from tensorflow.python.framework import test_util
label_desc: Use image node_id 10 for calling the HullWhiteTest obj's underlying member method code with example usage: obj.test_time_step_not_supplied() and returns: self