Specify the number of threads for parallel sort. (#8735)

* Specify the number of threads for parallel sort.

- Pass context object into argsort.
- Replace macros with inline functions.

tests/cpp/common/test_algorithm.cc

@@ -0,0 +1,35 @@
+/**
+ * Copyright 2020-2023 by XGBoost Contributors
+ */
+#include <gtest/gtest.h>
+#include <xgboost/context.h>  // Context
+#include <xgboost/span.h>
+
+#include <algorithm>  // is_sorted
+
+#include "../../../src/common/algorithm.h"
+
+namespace xgboost {
+namespace common {
+TEST(Algorithm, ArgSort) {
+  Context ctx;
+  std::vector<float> inputs{3.0, 2.0, 1.0};
+  auto ret = ArgSort<bst_feature_t>(&ctx, inputs.cbegin(), inputs.cend());
+  std::vector<bst_feature_t> sol{2, 1, 0};
+  ASSERT_EQ(ret, sol);
+}
+
+TEST(Algorithm, Sort) {
+  Context ctx;
+  ctx.Init(Args{{"nthread", "8"}});
+  std::vector<float> inputs{3.0, 1.0, 2.0};
+
+  Sort(&ctx, inputs.begin(), inputs.end(), std::less<>{});
+  ASSERT_TRUE(std::is_sorted(inputs.cbegin(), inputs.cend()));
+
+  inputs = {3.0, 1.0, 2.0};
+  StableSort(&ctx, inputs.begin(), inputs.end(), std::less<>{});
+  ASSERT_TRUE(std::is_sorted(inputs.cbegin(), inputs.cend()));
+}
+}  // namespace common
+}  // namespace xgboost