Avoid the use of size_t in the partitioner. (#10541)

- Avoid the use of size_t in the partitioner.
- Use `Span` instead of `Elem` where `node_id` is not needed.
- Remove the `const_cast`.
- Make sure the constness is not removed in the `Elem` by making it reference only.

size_t is implementation-defined, which causes issue when we want to pass pointer or span.

tests/cpp/common/test_partition_builder.cc

@@ -1,5 +1,5 @@
 /**
- * Copyright 2020-2023 by XGBoost contributors
+ * Copyright 2020-2024, XGBoost contributors
  */
 #include <gtest/gtest.h>
 
@@ -58,7 +58,7 @@ TEST(PartitionBuilder, BasicTest) {
   }
   builder.CalculateRowOffsets();
 
-  std::vector<size_t> v(*std::max_element(tasks.begin(), tasks.end()) * kBlockSize);
+  std::vector<bst_idx_t> v(*std::max_element(tasks.begin(), tasks.end()) * kBlockSize);
 
   for(size_t nid = 0; nid < kNodes; ++nid) {