Move nablaT_v2 + the flex-attention guard out of the Kandinsky5 DiT into
fastvideo/attention/backends/nabla.py with backend/metadata/builder/impl
classes, a NABLA_ATTN enum entry, and CUDA platform wiring that refuses to
silently fall back to dense attention when flex_attention is unavailable.
Add a generic default_backend parameter to the selector and LocalAttention —
a layer-level default that the global force and FASTVIDEO_ATTENTION_BACKEND
still override — so nabla checkpoints select the backend by default while
users keep env-var control. The DiT's sparse path now dispatches through a
dedicated LocalAttention (with a direct-flex fallback for standalone parity
tests without a forward context); verified bit-identical to the previous
inline flex call.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
