code: def _get_comm_key_from_devices(devices):
      """Return a key from a list of devices for collective calls.
  
      For example, if the tensors are on gpus 0, 1, 2, 3,
      then the key would be "0,1,2,3".
  
      Args:
          devices: a list of GPU device indices
  
      Returns:
          str: a string represents the key to query the communicator cache.
  
      """
      return ",".join([str(d) for d in devices])
  
imports: import logging
  import datetime
  import time
  import ray
  import cupy
  from ray.util.collective.const import ENV
  from ray.util.collective.collective_group import nccl_util
  from ray.util.collective.collective_group.base_collective_group import BaseGroup
  from ray.util.collective.const import get_store_name
  from ray.util.collective.types import AllReduceOptions, BarrierOptions, Backend, ReduceOptions, BroadcastOptions, AllGatherOptions, ReduceScatterOptions, SendOptions, RecvOptions
  from ray.util.collective.collective_group.cuda_stream import get_stream_pool
label_desc: Use image node_id 4 for calling a global function with example usage: _get_comm_key_from_devices(devices) and returns: str