fix(parallel): compile create_partition and divide_mpi_groups in non-MPI builds

create_partition and divide_mpi_groups were swallowed by an outer
#ifdef __MPI block, so the serial fallback branch never compiled and
non-MPI linking failed with undefined reference to create_partition.
Close the guard after divide_pools and keep both functions outside;
divide_mpi_groups is pure arithmetic and is called unconditionally by
Parallel_K2D. Also refresh the stale availability note in the header.

Author: abacus_fixer

source/source_base/parallel_global.cpp

@@ -282,6 +282,7 @@ void Parallel_Global::divide_pools(const int& NPROC,
     }
     return;
 }
+#endif
 
 ParallelPartition Parallel_Global::create_partition(int world_nproc,
                                                     int my_rank,
@@ -427,5 +428,3 @@ void Parallel_Global::divide_mpi_groups(const int& procs,
         rank_in_group = (rank - extra_procs) % procs_in_group;
     }
 }
-
-#endif

source/source_base/parallel_global.h

@@ -38,9 +38,9 @@ void read_pal_param(int argc, char** argv, int& NPROC, int& NTHREAD_PER_PROC, in
  * expected to fill them in from the appropriate view before passing the
  * partition down.
  *
- * Note: The factory is only available under __MPI. The non-MPI build path
- * uses the default ParallelPartition constructor which already produces the
- * single-process trivial partition.
+ * Note: Under __MPI the factory splits the real communicators as described
+ * below. The non-MPI build path returns the default ParallelPartition
+ * object, which already represents the single-process trivial partition.
  *
  * @param[in] world_nproc Size of MPI_COMM_WORLD
  * @param[in] my_rank    Rank in MPI_COMM_WORLD