docstr: Test trajectory stream with an added margin.
code: def test_trajectory_slice_stream_margin(self):
      """Test trajectory stream with an added margin."""
      tr1 = rl_task.Trajectory(0)
      self._extend(tr1, new_observation=1)
      self._extend(tr1, new_observation=1)
      self._extend(
          tr1,
          new_observation=1,
          action=1,
          dist_inputs=2,
          reward=3,
          done=True,
      )
      task = rl_task.RLTask(
          DummyEnv(), initial_trajectories=[tr1], max_steps=9
      )
  
      # Stream of slices without the final state.
      stream1 = task.trajectory_slice_stream(
          max_slice_length=4, margin=3
      )
      got_done = False
      for _ in range(20):
          next_slice = next(stream1)
          self.assertEqual(next_slice.observation.shape, (4,))
          if next_slice.done[0]:
              # In the slice, first we have the last timestep in the actual
              # trajectory, so observation = 1.
              # Then comes the first timestep in the margin, which has the final
              # observation from the trajectory: observation = 1.
              # The remaining timesteps have 0 observations.
              np.testing.assert_array_equal(
                  next_slice.observation, [1, 1, 0, 0]
              )
              # In the margin, done = True and mask = 0.
              for i in range(1, next_slice.observation.shape[0]):
                  self.assertTrue(next_slice.done[i])
                  self.assertFalse(next_slice.mask[i])
              got_done = True
      # Assert that we got a done somewhere, otherwise the test is not triggered.
      # Not getting done has low probability (1/2^20) but is possible, flaky test.
      self.assertTrue(got_done)
  
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 25 for calling the TaskTest obj's underlying member method code with example usage: obj.test_trajectory_slice_stream_margin() without return types