Calculate base_score based on input labels for mae. (#8107)

Fit an intercept as base score for abs loss.

src/common/threading_utils.h

@@ -8,6 +8,7 @@
 #include <dmlc/omp.h>
 
 #include <algorithm>
+#include <cstdint>  // std::int32_t
 #include <limits>
 #include <type_traits>  // std::is_signed
 #include <vector>
@@ -253,7 +254,7 @@ inline int32_t OmpGetNumThreads(int32_t n_threads) {
  * MaxStackSize, it will be allocated inside the stack. Otherwise, it will be
  * heap-allocated.
  */
-template <typename T, size_t MaxStackSize>
+template <typename T, std::size_t MaxStackSize>
 class MemStackAllocator {
  public:
   explicit MemStackAllocator(size_t required_size) : required_size_(required_size) {
@@ -278,11 +279,23 @@ class MemStackAllocator {
   T& operator[](size_t i) { return ptr_[i]; }
   T const& operator[](size_t i) const { return ptr_[i]; }
 
+  auto data() const { return ptr_; }                   // NOLINT
+  auto data() { return ptr_; }                         // NOLINT
+  std::size_t size() const { return required_size_; }  // NOLINT
+
+  auto cbegin() const { return data(); }         // NOLINT
+  auto cend() const { return data() + size(); }  // NOLINT
+
  private:
   T* ptr_ = nullptr;
   size_t required_size_;
   T stack_mem_[MaxStackSize];
 };
+
+/**
+ * \brief Constant that can be used for initializing static thread local memory.
+ */
+std::int32_t constexpr DefaultMaxThreads() { return 128; }
 }  // namespace common
 }  // namespace xgboost