[train] Migrate FlashRL fork to use the latest vLLM version #1338
Description
Summary
The SkyRL + FlashRL integration is currently broken because the FlashRL fork we use: https://github.com/sumanthrh/vllm/tree/flashrl is using an outdated vLLM version. We need to migrate it to use vLLM version compatible with SkyRL (0.16.0 and above)