Fix OPSM CP reduction and clean up interfaces

[cklxx]

Summary

• refactor OPSM masking to use explicit input builders and avoid context-parallel double counting
• compute OPSM-only CP KLs without autograd, enforcing strict zip length checks in critical paths
• update FSDP actor usage and guard CP reductions with required process groups

Testing

• ruff check .
• pytest (fails: environment cannot import slime module during test discovery)

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Codex Task