[mypy] Enable type checking for test directory (#5017)

tests/distributed/test_pynccl.py

@@ -1,5 +1,6 @@
 import multiprocessing
 import os
+from typing import Dict, List
 
 import pytest
 import torch
@@ -17,9 +18,9 @@ from vllm.utils import update_environment_variables
 
 def distributed_run(fn, world_size):
     number_of_processes = world_size
-    processes = []
+    processes: List[multiprocessing.Process] = []
     for i in range(number_of_processes):
-        env = {}
+        env: Dict[str, str] = {}
         env['RANK'] = str(i)
         env['LOCAL_RANK'] = str(i)
         env['WORLD_SIZE'] = str(number_of_processes)