Beating FP16 with 4-bit Weights: A Portable W4A16 GEMM in Triton

I wrote a 4-bit weight-only GEMM in pure Triton. The fast W4A16 kernels are all CUDA, so this one runs on NVIDIA and AMD. It beats cuBLAS FP16 by 1.1 to 1.3x in the decode regime, and the road there was mostly me being wrong about memory bandwidth.