code: def test_value_error_high_without_syncs(self):
      train_model = self._model_fn(mode="train")
      eval_model = self._model_fn(mode="eval")
      train_task = value_tasks.ValueTrainTask(
          self._trajectory_batch_stream,
          optimizer=opt.Adam(),
          lr_schedule=lr_schedules.constant(1e-3),
          advantage_estimator=advantages.td_k(
              gamma=self._task.gamma, margin=1
          ),
          model=train_model,
          target_model=eval_model,
          # Synchronize just once, at the end of training.
          sync_at=(lambda step: step == 100),
      )
      loop = training.Loop(
          model=train_model,
          eval_model=eval_model,
          tasks=[train_task],
      )
  
      # Assert that before training, the error is high.
      error_before = self._value_error(train_task.value)
      self.assertGreater(error_before, 2.0)
  
      loop.run(n_steps=100)
  
      # Assert that after training, the error is smaller, but still high.
      error_after = self._value_error(train_task.value)
  
      self.assertLess(error_after, 2.0)
      self.assertGreater(error_after, 0.8)
  
file_path: trax/trax/rl/value_tasks_test.py
imports: from absl.testing import absltest
  import numpy
  from trax import layers
  from trax import models
  from trax import optimizers
  from trax.rl import advantages
  from trax.rl import distributions
  from trax.rl import policy_tasks
  from trax.rl import task
  from trax.rl import value_tasks
  from trax.supervised import lr_schedules
  from trax.supervised import training
label_desc: Use image node_id 4 for calling the ValueTasksTest obj's underlying member method code with example usage: obj.test_value_error_high_without_syncs() without return types