docstr: Test that last_observation is the one passed in the last extend().
code: def test_nonempty_trajectory_last_observation(self):
      """Test that last_observation is the one passed in the last extend()."""
      tr = rl_task.Trajectory(observation=123)
      for _ in range(5):
          self._extend(tr)
      self._extend(tr, new_observation=321)
      self.assertEqual(tr.last_observation, 321)
  
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 5 for calling the TaskTest obj's underlying member method code with example usage: obj.test_nonempty_trajectory_last_observation() without return types