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

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• 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.

[NicoGrande]

LGTM! Could you share the logs for your test run?

[richjames0]

lgtm