Fixed "LoopEvent" handling in Loops and Branches

The original cel did not properly handle nor implement LoopEvents, leading to an error when the random number triggered the loop.

In this minimal change, we properly use the LoopEvent class while remaining faithful to the original code.

unit2/llama-index/workflows.ipynb

@@ -146,13 +146,14 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 28,
+   "execution_count": null,
    "metadata": {},
    "outputs": [
     {
      "name": "stdout",
      "output_type": "stream",
      "text": [
+      "Bad thing happened\n",
       "Good thing happened\n"
      ]
     },
@@ -182,7 +183,7 @@
     "\n",
     "class MultiStepWorkflow(Workflow):\n",
     "    @step\n",
-    "    async def step_one(self, ev: StartEvent) -> ProcessingEvent | LoopEvent:\n",
+    "    async def step_one(self, ev: StartEvent | LoopEvent) -> ProcessingEvent | LoopEvent:\n",
     "        if random.randint(0, 1) == 0:\n",
     "            print(\"Bad thing happened\")\n",
     "            return LoopEvent(loop_output=\"Back to step one.\")\n",
@@ -191,7 +192,7 @@
     "            return ProcessingEvent(intermediate_result=\"First step complete.\")\n",
     "\n",
     "    @step\n",
-    "    async def step_two(self, ev: ProcessingEvent | LoopEvent) -> StopEvent:\n",
+    "    async def step_two(self, ev: ProcessingEvent) -> StopEvent:\n",
     "        # Use the intermediate result\n",
     "        final_result = f\"Finished processing: {ev.intermediate_result}\"\n",
     "        return StopEvent(result=final_result)\n",