Fix compiler warnings. (#8022)

- Remove/fix unused parameters
- Remove deprecated code in rabit.
- Update dmlc-core.

src/tree/gpu_hist/feature_groups.cu

@@ -29,7 +29,7 @@ FeatureGroups::FeatureGroups(const common::HistogramCuts& cuts, bool is_dense,
   bin_segments_h.push_back(0);
 
   const std::vector<uint32_t>& cut_ptrs = cuts.Ptrs();
-  int max_shmem_bins = shm_size / bin_size;
+  size_t max_shmem_bins = shm_size / bin_size;
   max_group_bins = 0;
 
   for (size_t i = 2; i < cut_ptrs.size(); ++i) {

src/tree/gpu_hist/histogram.cu

@@ -188,7 +188,7 @@ void BuildGradientHistogram(EllpackDeviceAccessor const& matrix,
   int device = 0;
   dh::safe_cuda(cudaGetDevice(&device));
   // opt into maximum shared memory for the kernel if necessary
-  int max_shared_memory = dh::MaxSharedMemoryOptin(device);
+  size_t max_shared_memory = dh::MaxSharedMemoryOptin(device);
 
   size_t smem_size = sizeof(typename HistRounding<GradientSumT>::SharedSumT) *
                      feature_groups.max_group_bins;

src/tree/gpu_hist/row_partitioner.cu

@@ -79,6 +79,7 @@ void RowPartitioner::SortPosition(common::Span<bst_node_t> position,
 
 void Reset(int device_idx, common::Span<RowPartitioner::RowIndexT> ridx,
            common::Span<bst_node_t> position) {
+  dh::safe_cuda(cudaSetDevice(device_idx));
   CHECK_EQ(ridx.size(), position.size());
   dh::LaunchN(ridx.size(), [=] __device__(size_t idx) {
     ridx[idx] = idx;
@@ -92,7 +93,7 @@ RowPartitioner::RowPartitioner(int device_idx, size_t num_rows)
   dh::safe_cuda(cudaSetDevice(device_idx_));
   ridx_ = dh::DoubleBuffer<RowIndexT>{&ridx_a_, &ridx_b_};
   position_ = dh::DoubleBuffer<bst_node_t>{&position_a_, &position_b_};
-  ridx_segments_.emplace_back(Segment(0, num_rows));
+  ridx_segments_.emplace_back(static_cast<size_t>(0), num_rows);
 
   Reset(device_idx, ridx_.CurrentSpan(), position_.CurrentSpan());
   left_counts_.resize(256);