Small cleanup for histogram routines. (#9427)

* Small cleanup for histogram routines.

- Extract hist train param from GPU hist.
- Make histogram const after construction.
- Unify parameter names.

src/common/threading_utils.h

@@ -136,7 +136,7 @@ class BlockedSpace2d {
 // Wrapper to implement nested parallelism with simple omp parallel for
 template <typename Func>
 void ParallelFor2d(const BlockedSpace2d& space, int nthreads, Func func) {
-  const size_t num_blocks_in_space = space.Size();
+  std::size_t n_blocks_in_space = space.Size();
   CHECK_GE(nthreads, 1);
 
   dmlc::OMPException exc;
@@ -144,11 +144,10 @@ void ParallelFor2d(const BlockedSpace2d& space, int nthreads, Func func) {
   {
     exc.Run([&]() {
       size_t tid = omp_get_thread_num();
-      size_t chunck_size =
-          num_blocks_in_space / nthreads + !!(num_blocks_in_space % nthreads);
+      size_t chunck_size = n_blocks_in_space / nthreads + !!(n_blocks_in_space % nthreads);
 
-      size_t begin = chunck_size * tid;
-      size_t end = std::min(begin + chunck_size, num_blocks_in_space);
+      std::size_t begin = chunck_size * tid;
+      std::size_t end = std::min(begin + chunck_size, n_blocks_in_space);
       for (auto i = begin; i < end; i++) {
         func(space.GetFirstDimension(i), space.GetRange(i));
       }