feat: (Gen AI) Create a sample for the evaluation/get_rouge_score section

[Valeriy-Burlaka]

Description

Adds a sample for the "Get rouge score" section of the model-reference/evaluation Vertex AI page.

Checklist

• [X] I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• [ ] README is updated to include all relevant information
• [X] Tests pass: nox -s py-3.8 && nox -s py-3.11
• [X] Lint pass: nox -s lint
• [ ] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [ ] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [ ] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• [ ] This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• [ ] Please merge this PR for me once it is approved

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Here is the summary of changes.

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- [generative_ai/evaluation/get_rouge_score.py:22](https://github.com/Valeriy-Burlaka/python-docs-samples/blob/8187ab64290702f44d3a9b9010cb3ca845e4f492/generative_ai/evaluation/get_rouge_score.py#L22), tag `generativeaionvertexai_evaluation_get_rouge_score`

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[Valeriy-Burlaka]

The py-3.11 build fails on CI because of a "...service out of capacity" error in a different test file:

=================================== FAILURES ===================================
_________________________ test_config_response_schema __________________________
Traceback (most recent call last):
  File "/workspace/generative_ai/.nox/py-3-11/lib/python3.11/site-packages/google/api_core/grpc_helpers.py", line 76, in error_remapped_callable
    return callable_(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspace/generative_ai/.nox/py-3-11/lib/python3.11/site-packages/grpc/_channel.py", line 1181, in __call__
    return _end_unary_response_blocking(state, call, False, None)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspace/generative_ai/.nox/py-3-11/lib/python3.11/site-packages/grpc/_channel.py", line 1006, in _end_unary_response_blocking
    raise _InactiveRpcError(state)  # pytype: disable=not-instantiable
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
grpc._channel._InactiveRpcError: <_InactiveRpcError of RPC that terminated with:
    status = StatusCode.RESOURCE_EXHAUSTED
    details = "Unable to submit request because the service is temporarily out of capacity. Try again later."
    debug_error_string = "UNKNOWN:Error received from peer ipv4:173.194.202.95:443 {grpc_message:"Unable to submit request because the service is temporarily out of capacity. Try again later.", grpc_status:8, created_time:"2024-09-26T16:15:43.406844736+00:00"}"
>