Remove unnecessary calls to iota. (dmlc#6797)

src/metric/auc.cu

@@ -219,8 +219,6 @@ float GPUMultiClassAUCOVR(common::Span<float const> predts, MetaInfo const &info
   /**
    * Create sorted index for each class
    */
-  auto d_sorted_idx = dh::ToSpan(cache->sorted_idx);
-  dh::Iota(d_sorted_idx, device);
   auto d_predts_t = dh::ToSpan(cache->predts_t);
   Transpose(predts, d_predts_t, n_samples, n_classes, device);
 
@@ -231,6 +229,7 @@ float GPUMultiClassAUCOVR(common::Span<float const> predts, MetaInfo const &info
   });
   // no out-of-place sort for thrust, cub sort doesn't accept general iterator. So can't
   // use transform iterator in sorting.
+  auto d_sorted_idx = dh::ToSpan(cache->sorted_idx);
   dh::SegmentedArgSort<false>(d_predts_t, d_class_ptr, d_sorted_idx);
 
   /**
@@ -447,10 +446,9 @@ GPURankingAUC(common::Span<float const> predts, MetaInfo const &info,
   /**
    * Sort the labels
    */
-  auto d_sorted_idx = dh::ToSpan(cache->sorted_idx);
   auto d_labels = info.labels_.ConstDeviceSpan();
 
-  dh::Iota(d_sorted_idx, device);
+  auto d_sorted_idx = dh::ToSpan(cache->sorted_idx);
   dh::SegmentedArgSort<false>(d_labels, d_group_ptr, d_sorted_idx);
 
   auto d_weights = info.weights_.ConstDeviceSpan();