Preserve order of saved updaters config. (#9355)

- Save the updater sequence as an array instead of object.
- Warn only once.

The compatibility is kept, but we should be able to break it as the config is not loaded
in pickle model and it's declared to be not stable.

src/data/simple_dmatrix.cc

@@ -21,8 +21,7 @@
 #include "xgboost/c_api.h"
 #include "xgboost/data.h"
 
-namespace xgboost {
-namespace data {
+namespace xgboost::data {
 MetaInfo& SimpleDMatrix::Info() { return info_; }
 
 const MetaInfo& SimpleDMatrix::Info() const { return info_; }
@@ -97,6 +96,10 @@ BatchSet<SparsePage> SimpleDMatrix::GetRowBatches() {
 BatchSet<CSCPage> SimpleDMatrix::GetColumnBatches(Context const* ctx) {
   // column page doesn't exist, generate it
   if (!column_page_) {
+    auto n = std::numeric_limits<decltype(Entry::index)>::max();
+    if (this->sparse_page_->Size() > n) {
+      error::MaxSampleSize(n);
+    }
     column_page_.reset(new CSCPage(sparse_page_->GetTranspose(info_.num_col_, ctx->Threads())));
   }
   auto begin_iter = BatchIterator<CSCPage>(new SimpleBatchIteratorImpl<CSCPage>(column_page_));
@@ -106,6 +109,10 @@ BatchSet<CSCPage> SimpleDMatrix::GetColumnBatches(Context const* ctx) {
 BatchSet<SortedCSCPage> SimpleDMatrix::GetSortedColumnBatches(Context const* ctx) {
   // Sorted column page doesn't exist, generate it
   if (!sorted_column_page_) {
+    auto n = std::numeric_limits<decltype(Entry::index)>::max();
+    if (this->sparse_page_->Size() > n) {
+      error::MaxSampleSize(n);
+    }
     sorted_column_page_.reset(
         new SortedCSCPage(sparse_page_->GetTranspose(info_.num_col_, ctx->Threads())));
     sorted_column_page_->SortRows(ctx->Threads());
@@ -427,5 +434,4 @@ SimpleDMatrix::SimpleDMatrix(RecordBatchesIterAdapter* adapter, float missing, i
 
   fmat_ctx_ = ctx;
 }
-}  // namespace data
-}  // namespace xgboost
+}  // namespace xgboost::data