docstr: Number of hypotheses in the list.
docstr_tok: ["Number","of","hypotheses","in","the","list","."]
code: def __len__(self):
      """
      Number of hypotheses in the list.
      """
      return len(self.beams)
  
imports: from abc import ABC, abstractmethod
  from collections import UserDict
  from typing import Optional, Tuple
  from .file_utils import add_start_docstrings
  from fastNLP.envs.imports import _NEED_IMPORT_TORCH
  from fastNLP.core.log import logger
label_desc: Use image node_id 2 for calling the BeamHypotheses obj's underlying member method code with example usage: obj.__len__() and returns: len