code: def _flatten_and_concat_variables(vs):
      """Flatten and concat variables to make a single flat vector variable."""
      return F.concat([F.flatten(v) for v in vs], axis=0)
  
imports: import collections
  import itertools
  from logging import getLogger
  import random
  import chainer
  import chainer.functions
  import numpy
  import chainerrl
  from chainerrl import agent
  from chainerrl.agents.ppo import _compute_explained_variance
  from chainerrl.agents.ppo import _make_dataset
  from chainerrl.agents.ppo import _make_dataset_recurrent
  from chainerrl.agents.ppo import _yield_subset_of_sequences_with_fixed_number_of_items
  from chainerrl.misc.batch_states import batch_states
label_desc: Use image node_id 2 for calling a global function with example usage: _flatten_and_concat_variables(vs) and returns: F