code: def gather_all(self):
      return self._trajectories
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from absl.testing import parameterized
  import numpy
  import tensorflow
  from tf_agents.drivers import test_utils
  from tf_agents.drivers import tf_driver
  from tf_agents.environments import batched_py_environment
  from tf_agents.environments import tf_py_environment
  from tf_agents.trajectories import trajectory
  from tf_agents.utils import nest_utils
  from tf_agents.utils import test_utils
label_desc: Use image node_id 3 for calling the MockReplayBufferObserver obj's underlying member method code with example usage: obj.gather_all() and returns: self