[Bug]: Prompt optimzer fails with Prompt template must have a placeholder named 'target'

### File Name

gemini/prompts/prompt_optimizer/vertex_ai_prompt_optimizer_sdk_long_prompt_optimization.ipynb

### What happened?

I was using this notebook to run the prompt optimizer.
But the vertex AI job is failing with the following error

### Relevant log output

```json
{"Error": "Error: the validation for the template failed: Prompt template must have a placeholder named 'target' See gs:///optimization/optimization_jobs/auto-prompt-design-job-j8qe48uy/error.json for more details.", "Stack trace": "Traceback (most recent call last):\n File \"/optimize_main.par/google3/cloud/ml/research/llm/instruction_optimization/optimize_main.py\", line 248, in _parse_and_validate_template\n raise app.UsageError(\nabsl.app.UsageError: Prompt template must have a placeholder named 'target'\n"}
```

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