MegatronPolicyWorker refactor #1593
Description
draft POC: #1592
We want this refactor to align closely with #1589. We'll follow the merge plan listed in #1589, breaking the POC into more manageable PRs:
- Refactor init: feat: refactor megatron init #1646
- Refactor data processing: feat: refactor megatron data utils #1651
- Refactor model forward, forward_backward, etc: feat: refactor mcore train/forward utilities #1654