code: def stack_filter():
      stack_filter = InputFilter()
      stack_filter.add_observation_filter(
          "observation",
          "stack",
          ObservationStackingFilter(4, stacking_axis=-1),
      )
      return stack_filter
  
file_path: coach/rl_coach/tests/filters/observation/test_observation_stacking_filter.py
imports: import os
  import sys
  import pytest
  import numpy
  from rl_coach.filters.observation.observation_stacking_filter import ObservationStackingFilter
  from rl_coach.spaces import ObservationSpace
  from rl_coach.core_types import EnvResponse
  from rl_coach.filters.filter import InputFilter
label_desc: Use image node_id 2 for calling a global function with example usage: stack_filter() and returns: stack_filter