[rollout] feat: add optional tags parameter to release()

[cavities12]

What does this PR do?

Adds an optional tags parameter to release() across all rollout backends (SGLang, vLLM, TRT-LLM), matching the existing resume(tags) API introduced in #1911.

release() currently always releases both kv_cache and weights. With this change, callers can selectively release ["weights"], ["kv_cache"], or both ["kv_cache", "weights"] (the default, preserving backward compatibility).

This completes the symmetry between resume(tags) and release(tags) that was left unfinished in #1911, enabling use cases like resyncing only weights after update_weights() without touching kv_cache.

Checklist Before Starting

• Search for similar PRs: https://github.com/verl-project/verl/pulls?q=is%3Apr+release+tags — [rollout] feat: Support Multi-stage Awake for SGLang #1911 added tag-based resume() but left release() hardcoded
• Format the PR title as [{modules}] {type}: {description}

Test

15 unit tests covering all 3 backends:

• Default (None) releases both — backward compatible
• Selective release (["weights"], ["kv_cache"])
• Unknown tag validation (ValueError)
• Edge cases (non-leader noop, free_cache_engine disabled)
• vLLM: ["weights"]-only raises NotImplementedError (sleep levels don't support it)

tests/workers/rollout/test_release_tags.py .... 15 passed in 0.03s

API and Usage Example

# Before (still works — backward compatible)
await rollout.release()  # releases both kv_cache and weights

# After — selective release
await rollout.release(tags=["weights"])    # release only weights
await rollout.release(tags=["kv_cache"])   # release only kv_cache

Design & Code Changes

File	Change
base.py	ABC signature: release(self, tags: list[str] | None = None)
sglang_rollout.py	Delegates tags to engine, validates unknown tags
vllm_rollout.py	Maps tags to sleep levels (1=kv_cache, 2=both), NotImplementedError for weights-only
trtllm_rollout.py	Resolves abstract "weights" to internal _WEIGHTS_TAGS, validates via set
test_release_tags.py	15 async tests across all 3 backends

All backends validate unknown tags with ValueError. Tags are normalized to set() to prevent duplicates.

Checklist Before Submitting

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• Add unit or end-to-end test(s) to the CI workflow to cover all the code.
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[gemini-code-assist]

Code Review

This pull request introduces an optional tags parameter to the release() method across SGLang, vLLM, and TRT-LLM backends, enhancing granular control over resource release while maintaining backward compatibility and including appropriate validation. A security audit confirmed that these changes do not introduce any high or critical security vulnerabilities. However, a significant maintainability concern has been identified regarding the testing strategy.

[ETOgaosion]

Thanks for your contribution!
I suggest split vllm, trtllm and sglang tests as diffferent function with arguments, and seperate them to vllm/sgl/trtllm CI test using args to control.
So that when rollout engines upgrade, we can know the API changes at time.