fa4-cute-runtime

Forward-only, SM100-family FlashAttention-4 CuTe DSL package used by FlashRT's production GROOT N1.7 and PI0.5 Thor runtimes. In addition to the standard D48/D72/D128 path, it includes the dedicated D256 2CTA forward kernel needed by PI0.5's PaliGemma encoder.

The package exposes flash_attn_func, flash_attn_varlen_func, and the preallocated forward_static CUDA Graph entry, including seqused_k for fixed-shape padded K/V. It keeps the vendored implementation in the private flashrt_fa4 namespace and vendors the required quack helper subset, so it cannot replace or mutate upstream flash_attn imports.

See CARD.md for usage and VALIDATION.md for the release gate.