Enable data parallelism in RL rollout

[xuefgu]

Description

Enable data parallelism in RL rollout

Tests

Manually tested with 1 trainer slices and 2 samplers slices (each with 2 model replicas).

Checklist

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• [X] I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• [X] I have necessary comments in my code, particularly in hard-to-understand areas.
• [X] I have run end-to-end tests tests and provided workload links above if applicable.
• [X] I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.