fix(testing): add CPU execution flags and standardize E2E checkpoint conversion tests

[RexBearIU]

Description

This PR fixes the E2E Hugging Face checkpoint conversion test suite when executed in CPU environments:

Key Changes

1. CPU Execution Flags:

   • Added hardware=cpu and skip_jax_distributed_system=True across the conversion test scripts (gemma4, gpt-oss, llama3.1, qwen3, qwen3-vl).
   • Prevents multi-host TPU initialization hangs when executing on single-host CPU pods.

2. Threshold Alignment:

   • gemma4-26b: Set --max_kl_div=0.10 to account for MoE router boundary sensitivity and 256k vocabulary BF16 tail accumulation on long sequences (peak single-token KL is 0.082, top tokens identical).
   • qwen3-vl-2b: Tightened --max_kl_div=0.03.

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Tests

• Verified on GKE CPU cluster across the E2E conversion test suite.

Checklist

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• 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.

[gemini-code-assist]

Code Review

This pull request updates several end-to-end TPU test scripts to download pre-computed golden logits from Google Cloud Storage instead of generating them on the fly or running Hugging Face models. It also standardizes the test execution by adding hardware=cpu and skip_jax_distributed_system=True parameters. The reviewer suggests using [ ! -s ... ] instead of [ ! -f ... ] when checking for the downloaded logit files to robustly handle potentially empty files from interrupted downloads.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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