Cleanup to prepare for using mmap pointer in external memory. (#9317)

- Update SparseDMatrix comment.
- Use a pointer in the bitfield. We will replace the `std::vector<bool>` in `ColumnMatrix` with bitfield.
- Clean up the page source. The timer is removed as it's inaccurate once we swap the mmap pointer into the page.

src/collective/nccl_device_communicator.cu

@@ -70,7 +70,7 @@ NcclDeviceCommunicator::~NcclDeviceCommunicator() {
 
 namespace {
 ncclDataType_t GetNcclDataType(DataType const &data_type) {
-  ncclDataType_t result;
+  ncclDataType_t result{ncclInt8};
   switch (data_type) {
     case DataType::kInt8:
       result = ncclInt8;
@@ -108,7 +108,7 @@ bool IsBitwiseOp(Operation const &op) {
 }
 
 ncclRedOp_t GetNcclRedOp(Operation const &op) {
-  ncclRedOp_t result;
+  ncclRedOp_t result{ncclMax};
   switch (op) {
     case Operation::kMax:
       result = ncclMax;