Deterministic result for element-wise/mclass metrics. (#7303)

Remove openmp reduction.

tests/cpp/metric/test_multiclass_metric.cc

@@ -4,6 +4,43 @@
 
 #include "../helpers.h"
 
+namespace xgboost {
+inline void CheckDeterministicMetricMultiClass(StringView name, int32_t device) {
+  auto lparam = CreateEmptyGenericParam(device);
+  std::unique_ptr<Metric> metric{Metric::Create(name.c_str(), &lparam)};
+
+  HostDeviceVector<float> predts;
+  MetaInfo info;
+  auto &h_labels = info.labels_.HostVector();
+  auto &h_predts = predts.HostVector();
+
+  SimpleLCG lcg;
+
+  size_t n_samples = 2048, n_classes = 4;
+  h_labels.resize(n_samples);
+  h_predts.resize(n_samples * n_classes);
+
+  {
+    SimpleRealUniformDistribution<float> dist{0.0f, static_cast<float>(n_classes)};
+    for (size_t i = 0; i < n_samples; ++i) {
+      h_labels[i] = dist(&lcg);
+    }
+  }
+
+  {
+    SimpleRealUniformDistribution<float> dist{0.0f, 1.0f};
+    for (size_t i = 0; i < n_samples * n_classes; ++i) {
+      h_predts[i] = dist(&lcg);
+    }
+  }
+
+  auto result = metric->Eval(predts, info, false);
+  for (size_t i = 0; i < 8; ++i) {
+    ASSERT_EQ(metric->Eval(predts, info, false), result);
+  }
+}
+}  // namespace xgboost
+
 inline void TestMultiClassError(int device) {
   auto lparam = xgboost::CreateEmptyGenericParam(device);
   lparam.gpu_id = device;
@@ -17,12 +54,12 @@ inline void TestMultiClassError(int device) {
                             {0.1f, 0.1f, 0.1f, 0.1f, 0.1f, 0.1f, 0.1f, 0.1f, 0.1f},
                             {0, 1, 2}),
               0.666f, 0.001f);
-
   delete metric;
 }
 
 TEST(Metric, DeclareUnifiedTest(MultiClassError)) {
   TestMultiClassError(GPUIDX);
+  xgboost::CheckDeterministicMetricMultiClass(xgboost::StringView{"merror"}, GPUIDX);
 }
 
 inline void TestMultiClassLogLoss(int device) {
@@ -44,6 +81,7 @@ inline void TestMultiClassLogLoss(int device) {
 
 TEST(Metric, DeclareUnifiedTest(MultiClassLogLoss)) {
   TestMultiClassLogLoss(GPUIDX);
+  xgboost::CheckDeterministicMetricMultiClass(xgboost::StringView{"mlogloss"}, GPUIDX);
 }
 
 #if defined(XGBOOST_USE_NCCL) && defined(__CUDACC__)