[worker] fix: create a new event loop if none exists

[ji-huazhong]

What does this PR do?

I am working on integrating transferqueue into verl. Specifically, we convert metadata into dataproto in the register method of single_controller/base/decorator.py/. In this step, asyncio.run(tq_client.async_get_data(metadata) is called to get the specific data.

If asyncio.run and asyncio.get_event_loop are called sequentially in the same thread, a RuntimeError: There is no current event loop in thread %r is thrown.

This PR fixes the above-mentioned issue.

Checklist Before Starting

• Search for similar PRs. Paste at least one query link here: ...
• Format the PR title as [{modules}] {type}: {description} (This will be checked by the CI)
  • {modules} include fsdp, megatron, sglang, vllm, rollout, trainer, ci, training_utils, recipe, hardware, deployment, ray, worker, single_controller, misc, perf, model, algo, env, tool, ckpt, doc, data
  • If this PR involves multiple modules, separate them with , like [megatron, fsdp, doc]
  • {type} is in feat, fix, refactor, chore, test
  • If this PR breaks any API (CLI arguments, config, function signature, etc.), add [BREAKING] to the beginning of the title.
  • Example: [BREAKING][fsdp, megatron] feat: dynamic batching

Test

For changes that can not be tested by CI (e.g., algorithm implementation, new model support), validate by experiment(s) and show results like training curve plots, evaluation results, etc.

API and Usage Example

Demonstrate how the API changes if any, and provide usage example(s) if possible.

# Add code snippet or script demonstrating how to use this

Design & Code Changes

Demonstrate the high-level design if this PR is complex, and list the specific changes.

Checklist Before Submitting

Important

Please check all the following items before requesting a review, otherwise the reviewer might deprioritize this PR for review.

• Read the Contribute Guide.
• Apply pre-commit checks: pre-commit install && pre-commit run --all-files --show-diff-on-failure --color=always
• Add / Update the documentation.
• Add unit or end-to-end test(s) to the CI workflow to cover all the code. If not feasible, explain why: ...
• Once your PR is ready for CI, send a message in the ci-request channel in the verl Slack workspace. (If not accessible, please try the Feishu group (飞书群).)

[gemini-code-assist]

Code Review

This pull request replaces the deprecated asyncio.get_event_loop().run_until_complete() with the more modern and safer asyncio.run(). This is a good practice as asyncio.run() handles the event loop lifecycle automatically. However, I've identified a potential issue related to the mixed usage of synchronous and asynchronous patterns in the ActorRolloutRefWorker class hierarchy, which could lead to runtime errors in the future. Please see my detailed comment.