[vllm] feat: add model saving func to dump vllm state_dict before and after update weights

[RobotGF]

What does this PR do?

Add a debug func to dump vllm model state_dict by set VERL_VLLM_WEIGHT_SAVE_DIR

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[gemini-code-assist]

Code Review

This pull request introduces a debugging feature to save the vLLM model's state dictionary to safetensors files before and after weight updates. This is controlled by the VERL_VLLM_WEIGHT_SAVE_DIR environment variable. The implementation is straightforward. My review includes a suggestion to make the feature more robust by handling cases where the safetensors library might not be installed, preventing potential crashes in worker processes.