Fix RL Training - vLLM sampler TypeError

[chiajunglien]

Description

This PR resolves critical failures in RL training pipelines. It restores a previously dropped fix for the MaxTextVllmSampler initialization.

DAG Error Log

Root Cause

• RL Rollout TypeError: Commit 4521fc5 accidentally overwrote the fix applied in 5bf4b88 due to a stale branch merge. It reintroduced the invalid converter=converter kwarg into the base tunix VllmSampler.__init__, triggering an immediate TypeError during RL graph initialization.

Solution

Rollout Logic: Removed the invalid converter argument from super().__init__ in MaxTextVllmSampler, restoring the logic without breaking the newly added Qwen3.5 routing.

Tests

Llama3.1-70b RL: https://cloudlogging.app.goo.gl/i7cbeR8hzSyt38s36

Checklist

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• 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.

[gemini-code-assist]

Code Review

This pull request updates the __init__ method in maxtext_vllm_rollout.py by removing the converter argument from the super().__init__ call and removing the self.converter = converter assignment. The reviewer notes that while removing converter from super().__init__ is correct, removing self.converter = converter will break weight conversion because the base class expects this attribute. A suggestion is provided to restore the assignment.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...c/maxtext/integration/vllm/maxtext_vllm_rollout.py	0.00%	1 Missing ⚠️

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