[Megatron] fix: compatible to mcore0.15

[ISEEKYAN]

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[gemini-code-assist]

Code Review

This pull request updates the code to be compatible with mcore0.15 by removing the pipeline_model_parallel_split_rank parameter from calls to mpu.initialize_model_parallel. This parameter appears to have been removed from the function's signature in the new version of megatron-core, so this change is necessary to prevent runtime errors. The change is applied consistently in ActorRolloutRefWorker, CriticWorker, and RewardModelWorker. The modifications are correct and look good to merge.