[2/n]decouple quantization implementation from vLLM dependency (#8112)

Co-authored-by: walker-ai <yiyun.wyt@antgroup.com>
Co-authored-by: leoneo <1320612015@qq.com>

sgl-kernel/csrc/gemm/gptq/qdq_8.cuh

@@ -0,0 +1,30 @@
+/*
+Copied from https://github.com/turboderp/exllamav2
+*/
+
+#ifndef _qdq_8_cuh
+#define _qdq_8_cuh
+
+#include "qdq_util.cuh"
+
+namespace sglang {
+namespace gptq {
+
+__forceinline__ __device__ void shuffle_8bit_4(uint32_t* q, int stride) {}
+
+__forceinline__ __device__ void
+dequant_8bit_8(const uint32_t q_0, const uint32_t q_1, half2 (&dq)[4], int stride, const uint32_t zero) {
+  half dqh[8];
+  for (int i = 0; i < 4; i++)
+    dqh[i] = dq_ns(exb(q_0, i * 8, 0xff), zero);
+  for (int i = 0; i < 4; i++)
+    dqh[i + 4] = dq_ns(exb(q_1, i * 8, 0xff), zero);
+
+  for (int i = 0; i < 4; i++)
+    dq[i] = __halves2half2(dqh[i * 2], dqh[i * 2 + 1]);
+}
+
+}  // namespace gptq
+}  // namespace sglang
+
+#endif