Support custom rollout-proxy TIS hooks in bypass mode#1912
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Allow use_rollout_logprobs and use_tis to be combined through a custom TIS hook, while keeping the builtin decoupled TIS semantics unchanged. Expose current_log_probs and advantages to custom TIS hooks so rollout-proxy corrections can rebuild PPO/GRPO pg_loss directly without double-applying the same ratio. Add a reference helper in examples/train_infer_mismatch_helper, update the customization docs, and fix the train-vs-rollout mismatch metric so it still reflects training-engine logprobs in bypass mode.
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Summary
This PR keeps the builtin TIS behavior unchanged, but makes
use_rollout_logprobs + use_tisavailable through a custom hook.Changes:
custom_tis_function_pathwhenuse_rollout_logprobsanduse_tisare enabled togethercurrent_log_probsandadvantagesto custom TIS hooksexamples.train_infer_mismatch_helper.mis.compute_rollout_proxy_pg_loss_with_cpMotivation
The builtin TIS path assumes train-vs-rollout decoupled correction semantics.
In bypass mode, the main PPO/GRPO ratio is already computed against rollout logprobs. Reusing the builtin post-hoc TIS weighting in that setting can unintentionally apply the same rollout ratio twice.
This change does not alter the default path. It only opens an explicit extension point for users who want rollout-proxy correction behavior in asynchronous or stale-rollout settings.
Notes