docstr: Test that the epoch index -1 means last epoch and updates to it.
code: def test_task_epochs_index_minusone(self):
      """Test that the epoch index -1 means last epoch and updates to it."""
      obs = np.zeros((2,))
      tr1 = rl_task.Trajectory(obs)
      self._extend(tr1, new_observation=obs, done=True)
      task = rl_task.RLTask(
          DummyEnv(), initial_trajectories=[tr1], max_steps=9
      )
      stream = task.trajectory_slice_stream(
          epochs=[-1], max_slice_length=1
      )
      next_slice = next(stream)
      np.testing.assert_equal(next_slice.observation, np.zeros((1, 2)))
      task.collect_trajectories(
          policy=(lambda _: (0, 0)), n_trajectories=1
      )
      next_slice = next(stream)
      np.testing.assert_equal(next_slice.observation, np.ones((1, 2)))
  
imports: import os
  from absl.testing import absltest
  import gym
  import numpy
  from trax import test_utils
  from trax.rl import task
label_desc: Use image node_id 20 for calling the TaskTest obj's underlying member method code with example usage: obj.test_task_epochs_index_minusone() without return types