[recipe] feat: add multiturn scripts for vllm backend; fix progess bar in dapo

[jiaqiw09]

What does this PR do?

• Add example scirpt to run mutip-turn grpo in vllm and fsdp
• fix progressbar in dapo trainer
  • When enable_filter is enabled, DAPO runs multiple batch inferences before each actor update, but the progress bar advances once per inference—mismatching the true training step count and leading to confusion.

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[gemini-code-assist]

Code Review

This pull request introduces a new example script for multi-turn GRPO training using the vLLM backend and FSDP, and also includes a fix for a progress bar issue in the DAPO trainer. My review identifies critical issues in the new shell script where undefined variables are used, which would cause execution failures. I've provided comments to address these. The fix in the DAPO trainer appears correct and resolves the described problem of the progress bar advancing incorrectly.