class_docstr: Wrapper that stacks observations along a new final axis.
class_docstr_tok: ["Wrapper","that","stacks","observations","along","a","new","final","axis","."]
code: def _process_timestep(
      self, timestep: dm_env.TimeStep
  ) -> dm_env.TimeStep:
      observation = tree.map_structure(
          lambda stacker, x: stacker.step(x),
          self._stackers,
          timestep.observation,
      )
      return timestep._replace(observation=observation)
  
file_path: acme/acme/wrappers/frame_stacking.py
imports: import collections
  from acme import types
  from acme.wrappers import base
  import dm_env
  from dm_env import specs
  import numpy
  import tree
label_desc: Use image node_id 2 for calling the FrameStackingWrapper obj's underlying member method code with example usage: obj._process_timestep(timestep) and returns: timestep