The H3 dense self-attention call always ran on FlashAttention. On GB200,
cuDNN's fused SDPA engine is a few percent faster in-model at 37k-109k
tokens, but the ATen op has no shape metadata: it always reads inputs as
[B, H, S, D], so handing it contiguous [B, S, H, D] is silently misread.
Add FASTVIDEO_MINIMAX_H3_DENSE_ATTN=flash|cudnn (default flash) and route
the dense, unmasked, inference-only call through a helper that presents
BHSD strided views. Training calls, compiled regions, head sizes >128,
non-contiguous inputs, VSA blocks and NVFP4 stay on their existing paths.
FA4_* tuning and the default behaviour are unchanged.
Aryan Kumar
2ac6336b57