code: def register_subsearcher(name, subsearcher_cls):
      
      if not issubclass(subsearcher_cls, BaseSubSearcher):
          raise ValueError(
              '{} is not a subclass of BaseSubSearcher'.format(subsearcher_cls))
      registered_subsearchers[name] = subsearcher_cls
docs: register a subsearcher, the `name` is a key used for searching subsearchers.
      if the subsearcher named `name` already exists, then it's will overrite the old subsearcher.