class_docstr: Placeholder Transformer Agent.
  
  Placeholder class, which just throws an error telling the user to specify whether
  they want the ranker or the generator.
code: def __init__(self, opt, shared=None):
      raise RuntimeError(
          "`--model transformer` is not a valid choice. Please select either "
          "`--model transformer/ranker` or `--model transformer/generator"
      )
  
imports: from typing import Optional
  from parlai.core.params import ParlaiParser
  from parlai.core.opt import Opt
  from parlai.core.agents import Agent
  from parlai.utils.torch import padded_3d
  from parlai.core.torch_classifier_agent import TorchClassifierAgent
  from parlai.core.torch_ranker_agent import TorchRankerAgent
  from parlai.core.torch_generator_agent import TorchGeneratorAgent
  from parlai.utils.misc import recursive_getattr, warn_once
  from parlai.utils.logging import logging
  from parlai.utils.fsdp import should_use_fsdp
  from .modules import TransformerMemNetModel, TransformerGeneratorModel, TransformerLinearWrapper
  import torch
label_desc: Use image node_id 1 to create a new Transformer object from inherited base classes: Agent with example: obj = Transformer(opt, shared)