Add gemma3-4b E2E test scripts for pre-training and post-training

[SurbhiJainUSC]

Description

This PR adds E2E scripts for pre-training and post-training, including bi-directional checkpoint conversion for Gemma3-4b.

Tests

Pre-Training:

export RUN_ID=$(date +%Y-%m-%d-%H-%M)
bash tests/end_to_end/tpu/gemma3/4b/test_gemma3_to_mt.sh $RUN_ID
bash tests/end_to_end/tpu/gemma3/4b/test_gemma3.sh $RUN_ID

SFT:

export RUN_ID=$(date +%Y-%m-%d-%H-%M)
bash tests/end_to_end/tpu/gemma3/4b/test_gemma3_to_mt.sh $RUN_ID
bash tests/end_to_end/tpu/gemma3/4b/test_gemma3_sft.sh $RUN_ID

RL:

export RUN_ID=$(date +%Y-%m-%d-%H-%M)
bash tests/end_to_end/tpu/gemma3/4b/test_gemma3_to_mt.sh $RUN_ID
bash tests/end_to_end/tpu/gemma3/4b/test_gemma3_rl.sh $RUN_ID

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[hengtaoguo]

I noticed in this script it didn't have to_maxtext run. Is this intended?

[hengtaoguo]

This is correct, thanks for adding