Initial support for multi-target tree. (#8616)

* Implement multi-target for hist.

- Add new hist tree builder.
- Move data fetchers for tests.
- Dispatch function calls in gbm base on the tree type.

src/metric/rank_metric.cu

@@ -145,7 +145,6 @@ PackedReduceResult NDCGScore(Context const *ctx, MetaInfo const &info,
   auto d_predt = linalg::MakeTensorView(ctx, predt.ConstDeviceSpan(), predt.Size());
 
   auto d_group_ptr = p_cache->DataGroupPtr(ctx);
-  auto n_groups = info.group_ptr_.size() - 1;
 
   auto d_inv_idcg = p_cache->InvIDCG(ctx);
   auto d_sorted_idx = p_cache->SortedIdx(ctx, d_predt.Values());
@@ -171,7 +170,6 @@ PackedReduceResult MAPScore(Context const *ctx, MetaInfo const &info,
                             HostDeviceVector<float> const &predt, bool minus,
                             std::shared_ptr<ltr::MAPCache> p_cache) {
   auto d_group_ptr = p_cache->DataGroupPtr(ctx);
-  auto n_groups = info.group_ptr_.size() - 1;
   auto d_label = info.labels.View(ctx->gpu_id).Slice(linalg::All(), 0);
 
   predt.SetDevice(ctx->gpu_id);