class_docstr: An abstract class representing subcommands for allennlp.run.
  If you wanted to (for example) create your own custom `special-evaluate` command to use like
  
  `allennlp special-evaluate ...`
  
  you would create a `Subcommand` subclass and then pass it as an override to
  [`main`](#main).
code: def add_subparser(
      self, parser: argparse._SubParsersAction
  ) -> argparse.ArgumentParser:
      raise NotImplementedError
  
imports: import argparse
  from typing import Callable, Dict, Optional, Type, TypeVar
  from allennlp.common import Registrable
label_desc: Use image node_id 1 for calling the Subcommand obj's underlying member method code with example usage: obj.add_subparser(parser) without return types