Convert labels into tensor. (#7456)

* Add a new ctor to tensor for `initilizer_list`.
* Change labels from host device vector to tensor.
* Rename the field from `labels_` to `labels` since it's a public member.

tests/cpp/metric/test_elementwise_metric.cc

@@ -17,7 +17,7 @@ inline void CheckDeterministicMetricElementWise(StringView name, int32_t device)
 
   HostDeviceVector<float> predts;
   MetaInfo info;
-  auto &h_labels = info.labels_.HostVector();
+  auto &h_labels = info.labels.Data()->HostVector();
   auto &h_predts = predts.HostVector();
 
   SimpleLCG lcg;