class_docstr: Responsible for instantiating the Rendezvous server.
  
  Args:
      settings: Horovod Settings object.
class_docstr_tok: ["Responsible","for","instantiating","the","Rendezvous","server",".","Args",":","settings",":","Horovod","Settings","object","."]
code: def node_id_string(self) -> str:
      return ",".join(
          [
              f"{node_id}:{len(ranks)}"
              for node_id, ranks in self.node_id_by_rank.items()
          ]
      )
  
imports: import ray
  from ray.util.placement_group import get_current_placement_group
  from collections import defaultdict
  from dataclasses import dataclass, asdict
  import os
  from typing import Dict, Callable, Any, Optional, List
  import logging
  import ray.exceptions
  from horovod.ray.adapter import Adapter, BaseParams
  from horovod.runner.common.util import secret, timeout, hosts
  from horovod.runner.http.http_server import RendezvousServer
  from horovod.ray.utils import detect_nics, nics_to_env_var, map_blocking
  from horovod.ray.strategy import ColocatedStrategy, PGStrategy
  from horovod.ray.elastic_v2 import ElasticParams
label_desc: Use image node_id 4 for calling the Coordinator obj's underlying member method code with example usage: obj.node_id_string() and returns: str