[rollout] feat: use rollout and validate parallel process

[chenjiaoAngel]

What does this PR do?

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validate process and rollout process can be paralled in fully-async mode. This can reduce rollouter ilde and improve timing_s/gen speed when in short-long response_length case.

This is below profiler: (qwen2.5-Math-7b)

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we can use dapo_7b_math_fsdp2_8_8.sh, and add +async_training.parallel_validate_and_rollout=True command

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[gemini-code-assist]

Code Review

This pull request introduces parallel processing for validation in the FullyAsyncRollouter by using a ThreadPoolExecutor. This allows validation to run asynchronously, controlled by the parallel_validate_and_rollout configuration. The changes refactor the validation logic to support both synchronous and asynchronous execution paths. My review identifies a critical issue with unhandled exceptions in a fire-and-forget asyncio task, which could lead to silent failures. I have also pointed out several high-severity issues related to Python best practices, such as placing imports inside methods, accessing private attributes, using non-idiomatic boolean comparisons, and including non-English comments, all of which impact code quality and maintainability.