Shard - getting to 10× KV cache compression

TL;DR. Shard is a drop-in HuggingFace Cache that makes Llama-3.1-8B's KV memory about 10× smaller at 8K context (11× at 32K) without measurable hits to NIAH or LongBench. It started as a reimplementation of Google's TurboQuant[1], stalled around 4×, and ended up as a different design once we noticed K and V need different treatments: PCA plus int4 quantization on K (the matrix is effectively low-rank once you undo RoPE), and a Hadamard rotation plus vector quantization on V. Attention runs directly on the compressed K, no fp16 reconstruction. Code: krish1905/shard.