class_docstr: Defines a common interface of replay buffer.
  
  You can append transitions to the replay buffer and later sample from it.
  Replay buffers are typically used in experience replay.
docstr: Return the number of transitions in the buffer.
  
  Returns:
      Number of transitions in the buffer.
docstr_tok: ["Return","the","number","of","transitions","in","the","buffer",".","Returns",":","Number","of","transitions","in","the","buffer","."]
code: def __len__(self):
      """Return the number of transitions in the buffer.
  
      Returns:
          Number of transitions in the buffer.
      """
      raise NotImplementedError
  
imports: from abc import ABCMeta
  from abc import abstractmethod
  from abc import abstractproperty
  import itertools
  import numpy
  from chainerrl.misc.batch_states import batch_states
label_desc: Use image node_id 3 for calling the AbstractReplayBuffer obj's underlying member method code with example usage: obj.__len__() without return types