class_docstr: A mixin class for all pre-trained classes to inherit.
      
class_docstr_tok: ["A","mixin","class","for","all","pre-trained","classes","to","inherit","."]
code: def available_checkpoints(cls):
      return list(cls._MODEL2URL.keys())
  
imports: import os
  import sys
  from abc import ABCMeta, abstractmethod
  from pathlib import Path
  from texar.tf.data.data_utils import maybe_download
  from texar.tf.hyperparams import HParams
  from texar.tf.module_base import ModuleBase
label_desc: Use image node_id 1 for calling the PretrainedMixin obj's underlying member method code with example usage: obj.available_checkpoints(cls) and returns: list