[2/n] Migrate per_token_group_quant to torch stable ABI (#36058)

Signed-off-by: Mikayla Gawarecki <mikaylagawarecki@gmail.com>

csrc/libtorch_stable/quantization/vectorization.cuh

@@ -0,0 +1,31 @@
+#pragma once
+/**
+ * __device__ datatypes vectorized by 4
+ */
+
+// Include both AMD and NVIDIA fp8 types to avoid circular import
+#include <torch/headeronly/util/Float8_e4m3fnuz.h>
+#include <torch/headeronly/util/Float8_e4m3fn.h>
+
+namespace vllm {
+
+// Vectorization containers
+template <typename scalar_t, size_t vec_size>
+struct __align__(vec_size * sizeof(scalar_t)) vec_n_t {
+  scalar_t val[vec_size];
+};
+
+template <typename quant_type_t, size_t vec_size>
+struct __align__(vec_size * sizeof(quant_type_t)) q8_n_t {
+  static_assert(std::is_same_v<quant_type_t, int8_t> ||
+                std::is_same_v<quant_type_t, c10::Float8_e4m3fn> ||
+                std::is_same_v<quant_type_t, c10::Float8_e4m3fnuz>);
+  quant_type_t val[vec_size];
+};
+
+template <typename scalar_t>
+using vec4_t = vec_n_t<scalar_t, 4>;
+template <typename quant_type_t>
+using q8x4_t = q8_n_t<quant_type_t, 4>;
+
+}  // namespace vllm