[Refactor] Drop non-global aux-loss averaging mode#1965
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BREAKING CONFIG CHANGE: remove BalancingLossConfig.balancing_loss_global_average and ZLossConfig.z_loss_global_average. These switches only selected the per-rank (non-global) averaging mode, which is incompatible with reduce-sum gradients: the router/gate is a replicated param whose gradient is SUM-aggregated across the replicate group, so a full per-rank aux loss would inflate that gradient by world_size (the old FSDP AVG hid this by mean-reducing). No config or test in the repo ever set either switch to False. The finalize/accumulate branch now keys purely on dist.is_initialized(): distributed uses the global-average statistics; the single-process branch is kept (reference / eval run with the process group uninitialized) and is numerically the world-size-1 special case of the global branch (tokens_per_expert_global == tokens_per_expert_local, seqlen_global == valid_tokens), so its values are unchanged. Design doc S5.3 updated to state aux loss has only one averaging mode. Verified: single-process balancing/z-loss values bit-identical before/after; distributed balancing A/B, z-loss A/B and EP=1/EP=2 token-mean parity all pass.
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This PR (5/5) — breaking config change. Drops the
balancing_loss_global_average/z_loss_global_averageswitches and the non-global aux-loss mode, which is incompatible with reduce-sum (SUM-aggregated replicated router grad would inflate by world_size). Distributed = global-average; single-process is its W=1 special case.Full stack (merge bottom-up, under #1959):
reduce-sum-01-fsdp-helper— [FSDP] Add reduce-sum gradient reduction helperreduce-sum-02-split-logging— [Loss] Split logging loss from backward lossreduce-sum-03-switch-sum— [FSDP][Loss] Switch gradient reduction to SUMreduce-sum-04-remove-world-size— [Refactor] Remove unused world_size plumbingreduce-sum-05-drop-nonglobal— [Refactor] Drop non-global aux-loss averaging mode