graceful shutdown of vllm async

[f14-bertolotti]

Summary

This PR introduces a minimal refactoring to ensure that vLLM shuts down gracefully during cleanup. Without this change, users would encounter the following error at the end of an evaluation:

RuntimeError: There is no current event loop in thread 'MainThread'.

Additionally, I removed the @cache decorator, as it was not functioning correctly and resulted in:

TypeError: 'coroutine' object is not iterable