[sgd] hotfix example failure (#5297)

* hotfix

* Update train_example.py

python/ray/experimental/sgd/examples/train_example.py

@@ -25,7 +25,7 @@ if __name__ == "__main__":
     parser = argparse.ArgumentParser()
     parser.add_argument(
         "--redis-address",
-        required=True,
+        required=False,
         type=str,
         help="the address to use for Redis")
     parser.add_argument(
@@ -37,4 +37,4 @@ if __name__ == "__main__":
 
     import ray
     ray.init(redis_address=args.redis_address)
-    train_example(num_replicas=2, use_gpus=args.use_gpu)
+    train_example(num_replicas=2, use_gpu=args.use_gpu)