class_docstr: Mock Dictionary Agent which just implements indexing and txt2vec.
class_docstr_tok: ["Mock","Dictionary","Agent","which","just","implements","indexing","and","txt2vec","."]
code: def __len__(self):
      return 0
  
imports: from typing import Optional
  from parlai.core.params import ParlaiParser
  from parlai.core.opt import Opt
  from parlai.core.torch_agent import TorchAgent, Output
  import torch
  from parlai.core.agents import Agent
label_desc: Use image node_id 4 for calling the MockDict obj's underlying member method code with example usage: obj.__len__() and returns: int