[trainer] fix: change name for reward loop worker override

[ryxli]

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this was missed in the rebase 0102d04

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[gemini-code-assist]

Code Review

This pull request correctly renames the attribute check from reward_manager_worker to reward_loop_worker in the AgentLoopWorkerBase class. This change ensures consistency with the actual attribute name being assigned, aligning with the PR's objective to fix the name for the reward loop worker override.