code: def _mean_or_nan(xs):
      """Return its mean a non-empty sequence, numpy.nan for a empty one."""
      return np.mean(xs) if xs else np.nan
  
imports: import collections
  import copy
  from logging import getLogger
  import chainer
  from chainer import cuda
  import chainer.functions
  import numpy
  from chainerrl.agent import AttributeSavingMixin
  from chainerrl.agent import BatchAgent
  from chainerrl.misc.batch_states import batch_states
  from chainerrl.misc.copy_param import synchronize_parameters
  from chainerrl.replay_buffer import batch_experiences
  from chainerrl.replay_buffer import ReplayUpdater
label_desc: Use image node_id 1 for calling a global function with example usage: _mean_or_nan(xs) and returns: unknown