Add the public PaLM2 model.

[copybara-service[bot]]

Add the public PaLM2 model.

• The public pretrained PaLM2 model (text-bison-001) is accessible via lf.llms.Palm2.
• The public instruction-tuned PaLM2 model (chat-bison-001) is accessible via lf.llms.Palm2_IT.
• Renamed gemini.py to google_genai.py.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.69767% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 95.93%. Comparing base (6486b86) to head (4596777).

Files	Patch %	Lines
langfun/core/llms/google_genai.py	89.47%	2 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #164 +/- ## ========================================== - Coverage 96.00% 95.93% -0.07% ========================================== Files 55 55 Lines 4450 4482 +32 Branches 962 969 +7 ========================================== + Hits 4272 4300 +28 - Misses 90 92 +2 - Partials 88 90 +2 ```

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