class_docstr: Hugging Face GPT2 Agent.
  
  GPT2 is a multi-layer decoder-only Transformer.
  The decoder is initialized with pretrained weights from Hugging Face.
  Read more about this model here
  <https://huggingface.co/transformers/model_doc/gpt2.html>.
  
  GPT2 comes in five sizes: distilgpt2, small, medium, large, XL. Use the
  flag `--gpt2-size` to choose the size.
  
  If you are finetuning the Gpt2 agent as a dialogue agent, be sure
  to run `--add-special-tokens True`. To examine the performance of the
  agent out of the box, run with `--add-special-tokens False`, and make
  sure that the batch size is 1.
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code: def add_cmdline_args(
      cls, parser: ParlaiParser, partial_opt: Optional[Opt] = None
  ) -> ParlaiParser:
      agent = parser.add_argument_group("Gpt2 Args")
      agent.add_argument(
          "--model-name",
          type=str,
          default=None,
          help="Any GPT-2 model names.",
      )
      agent.add_argument(
          "--gpt2-size",
          type=str,
          default="small",
          choices=["small", "medium", "large", "xl", "distilgpt2"],
          help="Which size model to initialize.",
      )
      agent.add_argument(
          "--add-special-tokens",
          type="bool",
          default=True,
          help="Add special tokens (like PAD, etc.). If False, "
          "Can only use with batch size 1.",
      )
      agent.add_argument(
          "--add-start-token",
          type="bool",
          default=False,
          help="Add start tokens when finetuning.",
      )
      parser.set_defaults(
          text_truncate=768,
          label_truncate=256,
          dict_maxexs=0,  # skip building dictionary
      )
      super().add_cmdline_args(parser, partial_opt=partial_opt)
      warn_once(
          "WARNING: this model is in beta and the API is subject to change."
      )
      return agent
  
imports: from typing import Optional
  from parlai.core.params import ParlaiParser
  from parlai.core.opt import Opt
  import os
  import torch
  from parlai.agents.hugging_face.dict import Gpt2DictionaryAgent
  from parlai.core.torch_generator_agent import TorchGeneratorAgent, TorchGeneratorModel
  from parlai.utils.io import PathManager
  from parlai.utils.misc import warn_once
  from parlai.utils.torch import IdentityLayer, padded_tensor
label_desc: Use image node_id 1 for calling the Gpt2Agent obj's underlying member method code with example usage: obj.add_cmdline_args(cls, parser, partial_opt) and returns: agent