CUDA: add Q2_0 support (#25707)

ggml/src/ggml-cuda/dequantize.cuh

@@ -23,6 +23,26 @@ static __device__ __forceinline__ void dequantize_q1_0(const void * vx, const in
     v.y = (2*bit_1 - 1) * d;
 }
 
+static __device__ __forceinline__ void dequantize_q2_0(const void * vx, const int64_t ib, const int iqs, float2 & v){
+    const block_q2_0 * x = (const block_q2_0 *) vx;
+
+    const float d = x[ib].d;
+
+    // Q2_0: 2 bits per element, 4 elements per byte.
+    // Stored code c in {0,1,2,3} maps to symbol s = c - 1 in {-1, 0, +1, +2}.
+    const int byte_index_0 = iqs / 4;
+    const int bit_offset_0 = (iqs % 4) * 2;
+
+    const int byte_index_1 = (iqs + 1) / 4;
+    const int bit_offset_1 = ((iqs + 1) % 4) * 2;
+
+    const int c0 = (x[ib].qs[byte_index_0] >> bit_offset_0) & 0x3;
+    const int c1 = (x[ib].qs[byte_index_1] >> bit_offset_1) & 0x3;
+
+    v.x = (c0 - 1) * d;
+    v.y = (c1 - 1) * d;
+}
+
 static __device__ __forceinline__ void dequantize_q4_0(const void * vx, const int64_t ib, const int iqs, float2 & v){
     const block_q4_0 * x = (const block_q4_0 *) vx;