metal : improve dequantize precision to match CPU (llama/4836)

ggml-metal.metal

@@ -3841,8 +3841,8 @@ void dequantize_q3_K(device const block_q3_K *xb, short il, thread type4x4 & reg
     uint16_t scale_2 = scales[il%8], scale_1 = scales[8 + il%4];
     int16_t  dl_int = (il/4)&1 ? (scale_2&kmask2) | ((scale_1&kmask1) << 2)
                                : (scale_2&kmask2) | ((scale_1&kmask1) << 4);
-    half dl = il<8 ? d_all * (dl_int - 32.h) : d_all * (dl_int / 16.h - 32.h);
-    const half ml = 4.h * dl;
+    float dl = il<8 ? d_all * (dl_int - 32.f) : d_all * (dl_int / 16.f - 32.f);
+    const float ml = 4.f * dl;
 
     il = (il/2) & 3;
     const half    coef = il>1 ? (il>2 ? 1/64.h : 1/16.h) : (il>0 ? 1/4.h : 1.h);
@@ -3909,7 +3909,7 @@ void dequantize_q5_K(device const block_q5_K *xb, short il, thread type4x4 & reg
     uint8_t ul = 1 << (il/2);
     il = il & 3;
     const uchar2 sc = get_scale_min_k4_just2(is, il/2, xb->scales);
-    const float d = il < 2 ? xb->d : xb->d / 16.h;
+    const float d = il < 2 ? xb->d : xb->d / 16.f;
     const float min = xb->dmin;
     const float dl = d * sc[0];
     const float ml = min * sc[1];
@@ -3942,17 +3942,17 @@ void dequantize_q6_K(device const block_q6_K *xb, short il, thread type4x4 & reg
 #if QK_K == 256
     ql = ql + 64*(il/8) + 32*((il/2)&1) + 16*(il&1);
     qh = qh + 32*(il/8) + 16*(il&1);
-    half sc = scales[(il%2) + 2 * ((il/2))];
+    float sc = scales[(il%2) + 2 * ((il/2))];
     il = (il/2) & 3;
 #else
     ql = ql + 16 * (il&1);
-    half sc = scales[il];
+    float sc = scales[il];
 #endif
     const uint16_t  kmask1 = il>1 ? (il>2 ? 192 : 48) : (il>0 ? 12 : 3);
     const uint16_t  kmask2 = il>1 ? 0xF0              : 0x0F;
-    const half        coef = il>1 ? 1.f/16.h          : 1.h;
-    const half ml = d_all * sc * 32.h;
-    const half dl = d_all * sc * coef;
+    const float       coef = il>1 ? 1.f/16.f          : 1.f;
+    const float ml = d_all * sc * 32.f;
+    const float dl = d_all * sc * coef;
     for (int i = 0; i < 16; ++i) {
         const half q = il&1 ? ((ql[i] & kmask2) | ((qh[i] & kmask1) << 2))
                             : ((ql[i] & kmask2) | ((qh[i] & kmask1) << 4));