@rvarm1 the printed error in my local test is indeed cannot pickle 'TensorPipeRpcBackendOptions' object. In the original code, pickling TensorPipeRpcBackendOptions happens before initializing RPC or gloo. So I suspect @matrix was hitting a different error, but I cannot reproduce that error locally.
Traceback (most recent call last):
  File "tmp1.py", line 129, in <module>
    mp.spawn(workers_init_rpc, args=(world_size, options), nprocs=3, join=True)
  File "/raid/shenli/pytorch/torch/multiprocessing/spawn.py", line 230, in spawn
    return start_processes(fn, args, nprocs, join, daemon, start_method='spawn')
  File "/raid/shenli/pytorch/torch/multiprocessing/spawn.py", line 179, in start_processes
    process.start()
  File "/private/home/shenli/local/miniconda/envs/torchdev/lib/python3.8/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
  File "/private/home/shenli/local/miniconda/envs/torchdev/lib/python3.8/multiprocessing/context.py", line 284, in _Popen
    return Popen(process_obj)
  File "/private/home/shenli/local/miniconda/envs/torchdev/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/private/home/shenli/local/miniconda/envs/torchdev/lib/python3.8/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/private/home/shenli/local/miniconda/envs/torchdev/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 47, in _launch
    reduction.dump(process_obj, fp)
  File "/private/home/shenli/local/miniconda/envs/torchdev/lib/python3.8/multiprocessing/reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
TypeError: cannot pickle 'TensorPipeRpcBackendOptions' object