added a multiturn search based on Vertex AI Search (#11885)

Replace this entire comment with:
- **Description:** Added a retriever based on multi-turn Vertex AI
Search
  - **Twitter handle:** lkuligin

docs/docs/integrations/retrievers/google_vertex_ai_search.ipynb

@@ -161,7 +161,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "from langchain.retrievers import GoogleVertexAISearchRetriever\n",
+    "from langchain.retrievers import GoogleVertexAISearchRetriever, GoogleVertexAIMultiTurnSearchRetriever\n",
     "\n",
     "PROJECT_ID = \"<YOUR PROJECT ID>\"  # Set to your Project ID\n",
     "LOCATION_ID = \"<YOUR LOCATION>\"  # Set to your data store location\n",
@@ -247,6 +247,37 @@
     "for doc in result:\n",
     "    print(doc)"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "### Configure and use the retrieve for multi-turn search"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Search with follow-ups is [based](https://cloud.google.com/generative-ai-app-builder/docs/multi-turn-search) on generative AI models and it is different from the regular unstructured data search."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "retriever = GoogleVertexAIMultiTurnSearchRetriever(\n",
+    "    project_id=PROJECT_ID,\n",
+    "    location_id=LOCATION_ID,\n",
+    "    data_store_id=DATA_STORE_ID\n",
+    ")\n",
+    "\n",
+    "result = retriever.get_relevant_documents(query)\n",
+    "for doc in result:\n",
+    "    print(doc)"
+   ]
   }
  ],
  "metadata": {