vLLM sampler accesses attribute that does not exist

**Expected Behavior**

Running generation (for GRPO) with vLLM sampler should access attributes that actually exist.

**Actual Behavior**

Currently, attempting to use the vLLM sampler results in:
```
return self.llm.llm_engine.model_executor.driver_worker.model_runner
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'MultiprocExecutor' object has no attribute 'driver_worker'
```

AFAICT, there is no `driver_worker` attribute available. My guess is that there is a version mismatch between the expected and installed vLLM API?

**Steps to Reproduce the Problem**

N/A

**Environment**

- **OS:** Ubuntu
- **Project Version:** 0.1.4

**Checklist**

- [x] I have searched the existing issues for a similar bug report.
- [x] I have provided all the required information in the "Environment" section.
- [ ] I have provided a minimal, reproducible example.

**Would you like to help us fix it?**

With some guidance, I can help.