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.

src/objective/init_estimation.cc

@@ -1,5 +1,10 @@
+/**
+ * Copyright 2022-2023 by XGBoost contributors
+ */
 #include "init_estimation.h"
 
+#include <memory>                        // unique_ptr
+
 #include "../common/stats.h"             // Mean
 #include "../tree/fit_stump.h"           // FitStump
 #include "xgboost/base.h"                // GradientPair