Fix weighted samples in multi-class AUC. (#7300)

src/metric/auc.cu

@@ -291,7 +291,7 @@ float GPUMultiClassAUCOVR(common::Span<float const> predts, MetaInfo const &info
     // labels is a vector of size n_samples.
     float label = labels[idx % n_samples] == class_id;
 
-    float w = get_weight(i % n_samples);
+    float w = weights.empty() ? 1.0f : weights[d_sorted_idx[i] % n_samples];
     float fp = (1.0 - label) * w;
     float tp = label * w;
     return thrust::make_pair(fp, tp);