ReplaySSM: Cache SSM Inputs, Not State (Tri Dao)

Tri Dao's ReplaySSM changes SSM decoding so that recent inputs are cached and the recurrent state is rebuilt only when the buffer fills, rather than written to HBM every step. The output is mathematically unchanged, and rollback in speculative decoding becomes a buffer pointer move. Reported results in vLLM on B300 with NVFP4 models: up to 1.48x faster standard decoding, and 1.87–1.96x for speculative decoding, where vLLM's current path falls below standard decoding. These figures come from the author's own benchmarks, with no independent source given.