[Fix] Fix comm test (#1691)

Author: zhuohan123

tests/distributed/test_comm_ops.py

@@ -2,7 +2,7 @@
 
 Run `pytest tests/distributed/test_comm_ops.py --forked`.
 """
-from multiprocessing import Process
+from multiprocessing import Process, set_start_method
 
 import pytest
 import torch
@@ -70,6 +70,7 @@ def all_gather_test_worker(tensor_parallel_size: int, rank: int,
 @pytest.mark.parametrize("test_target",
                          [all_reduce_test_worker, all_gather_test_worker])
 def test_multi_process_tensor_parallel(tensor_parallel_size, test_target):
+    set_start_method("spawn", force=True)
     distributed_init_port = get_open_port()
     processes = []
     for rank in range(tensor_parallel_size):