feat: Add samples for the Tuning API reference doc

[nileshspringml]

Description

Add samples for the Tuning API reference doc: https://cloud.google.com/vertex-ai/generative-ai/docs/model-reference/tuning#python

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• README is updated to include all relevant information
• Tests pass: nox -s py-3.9 (see Test Environment Setup)
• Lint pass: nox -s lint (see Test Environment Setup)
• 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

[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• generative_ai/gemini_tuning.py:21, tag generativeaionvertexai_supervised_tuning
• generative_ai/gemini_tuning.py:58, tag generativeaionvertexai_get_supervised_tuning_job
• generative_ai/gemini_tuning.py:80, tag generativeaionvertexai_list_supervised_tuning_jobs
• generative_ai/gemini_tuning.py:101, tag generativeaionvertexai_cancel_supervised_tuning_job

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[holtskinner]

Test failure is unrelated to this change.