[core] fix workflow tests (ray-project#44994)

Fix workflow tests. The web service it used to compute fibonacci is
completely dead now.

Test:
- CI
- python
python/ray/workflow/examples/comparisons/prefect/compute_fib_workflow.py

Signed-off-by: can <can@anyscale.com>

python/ray/workflow/examples/comparisons/prefect/compute_fib_workflow.py

@@ -1,31 +1,36 @@
+import tempfile
+
 import ray
 from ray import workflow
-import requests
+from ray.actor import ActorHandle
+
 
+@ray.remote
+class FibonacciActor:
+    def __init__(self):
+        self.cache = {}
 
-def fibonacci(n):
-    assert n > 0
-    a, b = 0, 1
-    for _ in range(n - 1):
-        a, b = b, a + b
-    return b
+    def compute(self, n):
+        if n not in self.cache:
+            assert n > 0
+            a, b = 0, 1
+            for _ in range(n - 1):
+                a, b = b, a + b
+            self.cache[n] = b
+        return self.cache[n]
 
 
 @ray.remote
-def compute_large_fib(M: int, n: int = 1, fib: int = 1):
-    try:
-        next_fib = requests.post(
-            "https://nemo.api.stdlib.com/fibonacci@0.0.1/", data={"nth": n}
-        ).json()
-        assert isinstance(next_fib, int)
-    except AssertionError:
-        # TODO(suquark): The web service would fail sometimes. This is a workaround.
-        next_fib = fibonacci(n)
+def compute_large_fib(fibonacci_actor: ActorHandle, M: int, n: int = 1, fib: int = 1):
+    next_fib = ray.get(fibonacci_actor.compute.remote(n))
     if next_fib > M:
         return fib
     else:
-        return workflow.continuation(compute_large_fib.bind(M, n + 1, next_fib))
+        return workflow.continuation(
+            compute_large_fib.bind(fibonacci_actor, M, n + 1, next_fib)
+        )
 
 
 if __name__ == "__main__":
-    assert workflow.run(compute_large_fib.bind(100)) == 89
+    ray.init(storage=f"file://{tempfile.TemporaryDirectory().name}")
+    assert workflow.run(compute_large_fib.bind(FibonacciActor.remote(), 100)) == 89