Exception: process 0 terminated with exit code 1 when use torch.multiprocessing.spawn on GPUs

I got the error whenever I use the torch.multiprocessing.spawn to parallelize over multiple GPUs, including the code examples in Parallel and Distributed Training tutorials.

Exception: process 0 terminated with exit code 1 ​

Does anyone know the meaning of "terminating with exit code 1" (i.e., the reason why the process is terminated)?

The example in Pytorch DDP notes:

import torch
import torch.distributed as dist
import torch.multiprocessing as mp
import torch.nn as nn
import torch.optim as optim
from torch.nn.parallel import DistributedDataParallel as DDP


def example(rank, world_size):
    # create default process group
    dist.init_process_group("gloo", rank=rank, world_size=world_size)
    # create local model
    model = nn.Linear(10, 10).to(rank)
    # construct DDP model
    ddp_model = DDP(model, device_ids=[rank])
    # define loss function and optimizer
    loss_fn = nn.MSELoss()
    optimizer = optim.SGD(ddp_model.parameters(), lr=0.001)

    # forward pass
    outputs = ddp_model(torch.randn(20, 10).to(rank))
    labels = torch.randn(20, 10).to(rank)
    backward pass
    loss_fn(outputs, labels).backward()
    # update parameters
    optimizer.step()

def main():
    world_size = 2
    mp.spawn(example,
    args=(world_size,),
    nprocs=world_size,
    join=True)

if __name__=="__main__":
    main()

from Exception: process 0 terminated with exit code 1 when use torch.multiprocessing.spawn on GPUs