Stub the weight transfer in the AsyncGRPO epoch-stop test

[albertvillanova]

This PR fixes TestEpochStop::test_epoch_stop_is_fork_independent failing with ImportError: vLLM >= 0.22.0 is required to use WeightTransferClient.

FAILED tests/experimental/test_async_grpo_trainer.py::TestEpochStop::test_epoch_stop_is_fork_independent - ImportError: vLLM >= 0.22.0 is required to use WeightTransferClient. Install it with: pip install 'vllm>=0.22.0'

Motivation

The test builds a real AsyncGRPOTrainer but only passes rollout_worker, leaving weight_transfer unset. The trainer then constructs a real WeightTransferClient, which requires vLLM >= 0.22.0 and so cannot work in a test that has no vLLM server to talk to. Every other trainer-building test in the file already passes _StubWeightTransfer(); this one was missed.

The failure is not visible today because the class is gated behind is_ampere_or_newer() and the experimental CI runner is a T4, so the test is skipped. Moving that job to an L40S in #6741 makes it run for the first time.

Changes

• Pass weight_transfer=_StubWeightTransfer() when building the trainer in TestEpochStop._train

---

Note

^{Cursor Bugbot is generating a summary for commit e19e807. Configure here.}

[bot-ci-comment]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.