class_docstr: A helper mixin for testing Reverb adders.
  
  Note that any test inheriting from this mixin must also inherit from something
  that provides the Python unittest assert methods.
code: def num_episodes(self):
      info = self.client.server_info(1)[adders.DEFAULT_PRIORITY_TABLE]
      return info.num_episodes
  
imports: from typing import Any, Callable, Optional, Sequence, Tuple, TypeVar, Union
  from acme import specs
  from acme import types
  from acme.adders import base
  from acme.adders import reverb
  from acme.utils import tree_utils
  import dm_env
  import numpy
  import reverb
  import tensorflow
  import tree
  from absl.testing import absltest
label_desc: Use image node_id 4 for calling the AdderTestMixin obj's underlying member method code with example usage: obj.num_episodes() and returns: info