class_docstr: A wrapper which stacks previously seen frames.
class_docstr_tok: ["A","wrapper","which","stacks","previously","seen","frames","."]
code: def _transform_history_observations(self, frames):
      # Should be implemented if ever two StackWrappers are to be used together.
      raise NotImplementedError
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from six.moves import range
  from tensor2tensor.rl.envs.in_graph_batch_env import InGraphBatchEnv
  import tensorflow.compat.v1
label_desc: Use image node_id 6 for calling the StackWrapper obj's underlying member method code with example usage: obj._transform_history_observations(frames) without return types