Introduce flag to use pickle for serialization (#5805)

python/ray/tests/test_multi_node.py

@@ -551,3 +551,23 @@ print("success")
 
     # Make sure we can still talk with the raylet.
     ray.get(f.remote())
+
+
+@pytest.mark.parametrize(
+    "call_ray_start", ["ray start --head --num-cpus=1 --use-pickle"],
+    indirect=True)
+def test_use_pickle(call_ray_start):
+    address = call_ray_start
+
+    ray.init(address=address, use_pickle=True)
+
+    assert ray.worker.global_worker.use_pickle
+    x = (2, "hello")
+
+    @ray.remote
+    def f(x):
+        assert x == (2, "hello")
+        assert ray.worker.global_worker.use_pickle
+        return (3, "world")
+
+    assert ray.get(f.remote(x)) == (3, "world")