docstr: Test that last_observation is the one passed in __init__.
code: def test_empty_trajectory_last_observation(self):
      """Test that last_observation is the one passed in __init__."""
      tr = rl_task.Trajectory(observation=123)
      self.assertEqual(tr.last_observation, 123)
  
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 4 for calling the TaskTest obj's underlying member method code with example usage: obj.test_empty_trajectory_last_observation() without return types