[Bug]: import get_auth_token() is missing from vertex_ai_prompt_optimizer_sdk_custom_metric.ipynb

### File Name

https://github.com/GoogleCloudPlatform/generative-ai/blob/main/gemini/prompts/prompt_optimizer/vertex_ai_prompt_optimizer_sdk_custom_metric.ipynb

### What happened?

```
from tutorial.utils.vapo_lib import get_auth_token
```

is missing from the notebook

### Relevant log output

```shell

```

### Code of Conduct

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