NABLA block-sparse flex-attention backend (Kandinsky5 "nabla" checkpoints).
The block mask is data-dependent: nablaT_v2 mean-pools 64-token blocks of the fractal-ordered sequence, thresholds the softmaxed block map, and ORs it with a precomputed spatio-temporal-window (STA) mask carried on the attention metadata. The mask spans the full sequence, so this backend does not support sequence parallelism — use it via LocalAttention only.