UPSTREAM PR #18785: CUDA: Factor out and re-use `two_stage_warp_reduce` function#897

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upstream-PR18785-branch_ORippler-osimons/factor_out_two_stage_warp_reductions

Commits on Jan 12, 2026

CUDA: Refactor and expose two_stage_warp_reduce_* function
ORippler
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Use `two_stage_warp_reduce` also in softmax kernel, move smem out of it
ORippler
committed