feat(attention): emit forward LSE and harden matmul tuning - #428
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The flash forward's running (max, sum) state was discarded after the rescale; a FlashAttention backward needs the per-row log-sum-exp to recompute probabilities without materializing scores. New additive path: - BounceTile::store_row_lse writes m + ln(l) per absolute row through the whitebox fragment layout (the unit owning a row's first column writes; rows below the fully-masked threshold receive exactly -inf, matching the CPU reference convention), with a Tile-level dispatcher. - StageAttention::write_lse / GlobalAttention::execute_with_lse / BatchAttention::execute_with_lse thread a flat [batch * heads, seq_q] FP32 tensor through the existing layers; the global layer emits before the rescale consumes the state. - attention_with_lse entry point + BatchAttentionFamily:: launch_unchecked_with_lse + public launch_ref_with_lse / launch_attention_with_lse. Only the cmma (BlackboxAccelerated) routine emits LSE; the unit routine reports InvalidConfig loudly. Every existing signature is untouched.
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Summary
This PR absorbs the temporary compatibility-only draft #429 so CubeK has one upstream review thread.
The LSE output is flat
[batch * heads, seq_q], uses natural log, and writes negative infinity for fully masked rows. Existinglaunch_refand component signatures retain their behavior.The matmul key keeps anchored M/N/K buckets and bounded cardinality. It adds only two stride-legality classes per operand: 16-byte aligned or not aligned. This prevents a cached async-copy/TMA candidate from failing when a later raw shape in the same bucket has incompatible strides.
Dependency validation
mainis the direct base of this branch.tracel-ai/*repositories.Validation
cargo fmt --all -- --checkcargo test -p cubek-matmul strategy::tune_key --libcargo check -p cubek-attentioncargo test -p cubek-attention --no-runThe current Linux CI failure occurred in runner setup: Microsoft's Ubuntu apt repositories returned HTTP 403 before checkout/build tests began. Code-quality and documentation jobs passed.