Remove unused variables. (#9210)

- remove used variables.
- Remove signed comparison warnings.

src/collective/communicator-inl.h

@@ -252,7 +252,7 @@ inline AllgatherVResult<T> AllgatherV(std::vector<T> const &inputs,
 
   // Calculate input offsets (std::exclusive_scan).
   std::vector<std::size_t> offsets(all_sizes.size());
-  for (auto i = 1; i < offsets.size(); i++) {
+  for (std::size_t i = 1; i < offsets.size(); i++) {
     offsets[i] = offsets[i - 1] + all_sizes[i - 1];
   }
 

src/common/partition_builder.h

@@ -263,8 +263,7 @@ class PartitionBuilder {
   template <typename ExpandEntry>
   void PartitionByMask(const size_t node_in_set, std::vector<ExpandEntry> const& nodes,
                        const common::Range1d range, GHistIndexMatrix const& gmat,
-                       const common::ColumnMatrix& column_matrix, const RegTree& tree,
-                       const size_t* rid, BitVector const& decision_bits,
+                       const RegTree& tree, const size_t* rid, BitVector const& decision_bits,
                        BitVector const& missing_bits) {
     common::Span<const size_t> rid_span(rid + range.begin(), rid + range.end());
     common::Span<size_t> left = GetLeftBuffer(node_in_set, range.begin(), range.end());

src/tree/common_row_partitioner.h

@@ -67,7 +67,7 @@ class ColumnSplitHelper {
       const int32_t nid = nodes[node_in_set].nid;
       const size_t task_id = partition_builder_->GetTaskIdx(node_in_set, begin);
       partition_builder_->AllocateForTask(task_id);
-      partition_builder_->PartitionByMask(node_in_set, nodes, r, gmat, column_matrix, *p_tree,
+      partition_builder_->PartitionByMask(node_in_set, nodes, r, gmat, *p_tree,
                                           (*row_set_collection_)[nid].begin, decision_bits_,
                                           missing_bits_);
     });

src/tree/hist/evaluate_splits.h

@@ -298,7 +298,7 @@ class HistEvaluator {
     std::vector<CPUExpandEntry> all_entries(num_entries * world);
     std::vector<uint32_t> cat_bits;
     std::vector<std::size_t> cat_bits_sizes;
-    for (auto i = 0; i < num_entries; i++) {
+    for (std::size_t i = 0; i < num_entries; i++) {
       all_entries[num_entries * rank + i].CopyAndCollect(entries[i], &cat_bits, &cat_bits_sizes);
     }
     collective::Allgather(all_entries.data(), all_entries.size() * sizeof(CPUExpandEntry));
@@ -586,7 +586,7 @@ class HistMultiEvaluator {
     std::vector<uint32_t> cat_bits;
     std::vector<std::size_t> cat_bits_sizes;
     std::vector<GradientPairPrecise> gradients;
-    for (auto i = 0; i < num_entries; i++) {
+    for (std::size_t i = 0; i < num_entries; i++) {
       all_entries[num_entries * rank + i].CopyAndCollect(entries[i], &cat_bits, &cat_bits_sizes,
                                                          &gradients);
     }