Recompute SSM states instead of storing them for faster hybrid decoding

Tri Dao's post proposes loading SSM states, computing with them, and not storing them to make hybrid model decoding about 2x faster. The trick aims to ease the bottleneck from Gated-DeltaNet and Mamba states in hybrid models such as Qwen 3.5 and Nemotron Ultra, unlocking speculative decoding for SSMs.