[megatron] feat: Share actor and ref in LoRA

[HollowMan6]

Waiting for:

• [PEFT] feat: Add support for temporary disable adapter NVIDIA-NeMo/Megatron-Bridge#1811

For compute_ref_log_prob, we can do that by disabling lora layers temporarily for the forward pass, as base weight are frozen and only lora layers are trained.

This has already been supported in FSDP LoRA.

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[gemini-code-assist]

Code Review

This pull request introduces a valuable feature by enabling LoRA for Megatron models, allowing the actor and reference models to share weights. The implementation correctly uses a context manager to disable LoRA adapters for computing reference log probabilities. My review includes two high-severity suggestions: one to address duplicated code for determining lora_rank across multiple files, and another to fix a performance issue where entropy is unnecessarily calculated and then discarded.

[gemini-code-assist]

Code Review

This pull request refactors the LoRA configuration handling to support a nested structure and introduces a mechanism to share the actor and reference models in megatron_workers.py when using LoRA. This is achieved by temporarily disabling the LoRA adapter to compute reference log probabilities, which is a clean and efficient approach.

My main feedback is regarding the duplicated logic for determining the lora_rank across four different trainer files. I've left comments with suggestions to refactor this into a shared utility function for better maintainability. The rest of the changes look good.