fix: layer_cuda_shard_kvb scale offset for fmt=4 (multi-GPU, per JustVugg review)

The kv_b shard scale pointer used h0*(Q+V) which is correct for per-row
scales (fmt=2: one scale per row). For fmt=4 (grouped), there are ng
scales per row, so the offset must be h0*(Q+V)*ng. Without this, the
shard reads from the wrong scale position on multi-GPU, producing silent
corruption. Single-GPU is unaffected (no sharding).

Fix: const float *scale=l->kv_b.s+(int64_t)h0*(Q+V)*(gs>0?ng:1);

Refs #298

c/colibri.c

@@ -1647,7 +1647,7 @@ static void layer_cuda_shard_kvb(Layer *l,int H,int Q,int V){
     for(int d=0,h0=0;d<g_cuda_ndev;d++){
         int hn=H/g_cuda_ndev+(d<H%g_cuda_ndev),rows=hn*(Q+V);
         const void *part=weights+(int64_t)h0*(Q+V)*rb;
-        const float *scale=l->kv_b.s+(int64_t)h0*(Q+V);
+        const float *scale=l->kv_b.s+(int64_t)h0*(Q+V)*(l->kv_b.gs>0?(l->kv_b.I+l->kv_b.gs-1)/l->kv_b.gs:1);
         if(!coli_cuda_tensor_upload(&l->kv_b_shard[d],part,scale,l->kv_b.fmt,l->kv_b.I,rows,g_cuda_devices[d],l->kv_b.gs))return;
         l->shard_h0[d]=h0;l->shard_hn[d]=hn;l->n_kv_b_shard++;h0+=hn;
     }