Build grounded rag app with vertex with VertexAIRank example confusion

In the part `Step 4. Query in Real Time and Check Grounding` of `genai-on-vertex-ai/retrieval_augmented_generation/diy_rag_with_vertexai_apis/build_grounded_rag_app_with_vertex.ipynb`

The notebook mentioned that this step will "pass retrieved documents to the reranker API via the `VertexAIRank`" But I only saw the usage of `VertexAI` class and a basic retriever with the following code:

```python
retriever = vector_store.as_retriever(search_kwargs={"k": 5})

llm = VertexAI(model_name="gemini-1.5-pro-001", max_output_tokens=1024)
template = """
Answer the question based only on the following context:
{context}

Question:
{query}
"""
```

I feel a bit confused. If I am wrong or I miss something, please correct me! Thank you for your attention to this matter : )