feat(generative_ai): Add code completion example

[msampathkumar]

Add code completion examples with gemini flash model.

Please merge-on-approval!

Description

Fixes #

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

Checklist

• Tests pass: nox -s py-3.9 (see Test Environment Setup)
• Lint pass: nox -s lint (see Test Environment Setup)

[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• generative_ai/chat_completions/chat_completions_non_streaming_image.py:17, tag generativeaionvertexai_gemini_chat_completion_non_streaming_image
• generative_ai/chat_completions/chat_completions_non_streaming_text.py:17, tag generativeaionvertexai_gemini_chat_completion_non_streaming
• generative_ai/chat_completions/chat_completions_streaming_image.py:17, tag generativeaionvertexai_gemini_chat_completion_streaming_image
• generative_ai/chat_completions/chat_completions_streaming_text.py:17, tag generativeaionvertexai_gemini_chat_completion_streaming

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[gericdong]

Updates include:

• Specify auth scope
• Change file names to avoid class name collision
• Change function name to 'generate_text" to be more descriptive
• Change return to the response object. Returning strings won't work for streaming examples
• Remove nox config file from sub-directory. It will use the one in parent directory.
• Add # location="us-central1" in region tag so users are aware of the default location.
• Use OpenAI naming conversions for variables

[gericdong]

CI test is failing on test_gemini_chat_example which is irrelevant to this PR.

[msampathkumar]

Test cases failed with 500 error(google.api_core.exceptions.InternalServerError: 500. Rerunning the test cases.