[opd, fsdp, megatron] fix: enhance mem footprint for forward_kl_topk OPD

[dimjava]

What does this PR do?

Reduce VRAM and compute for OPD (forward_kl_topk + use_policy_gradient=False + use_task_rewards=False) by skipping redundant full-vocab log_probs and PPO-loss work when only the top-k distillation loss is needed.

Introduces a distillation_only flag (set in the trainer, consumed by actor engines) and scopes empty_cache() on Megatron to top-k distillation steps.

Target config: distillation.distillation_loss.loss_mode=forward_kl_topk with supervised distillation

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Test

pytest tests/workers/test_megatron_distillation_only_on_cpu.py -q
pytest tests/workers/test_distillation_topk_symmetry_on_cpu.py -q

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[gemini-code-assist]

Code Review

This pull request introduces a distillation_only flag to optimize memory usage and skip policy loss calculations during supervised top-k distillation. The changes span the PPO trainers, distillation loss calculations, and both FSDP and Megatron transformer engine implementations to conditionally omit log_probs computation. A potential issue was identified in the FSDP engine implementation (verl/workers/engine/fsdp/transformer_impl.py), where a destructive in-place modification of logits during log-probability calculation could corrupt the logits before they are processed by the distillation logits processor.

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[dimjava]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a "distillation_only" mode to optimize memory footprint and computation when performing supervised top-k distillation without policy gradients or task rewards. It skips the calculation, gathering, and propagation of log_probs across FSDP and Megatron engines, and adds corresponding unit tests to verify this behavior. Additionally, it empties the PyTorch cache before the Megatron optimizer step when top-k distillation is active to mitigate potential OOM issues.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.