feat(GenAI): Gemma2 samples for Model Garden deployments to Vertex AI endpoints

[rsamborski]

Description

Fixes b/365590892

Gemma2 samples for Model Garden deployments to Vertex AI endpoints
Tests are mocked for now until we make a decision if we want to have these models permanently deployed for testing purposes.

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)
• This sample adds a new sample directory, and I updated the CODEOWNERS file 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 2 region tags.

• gemma2/gemma2_predict_gpu.py:22, tag generativeaionvertexai_gemma2_predict_gpu
• gemma2/gemma2_predict_tpu.py:22, tag generativeaionvertexai_gemma2_predict_tpu

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