Use std::uint64_t for row index. (#10120)

- Use std::uint64_t instead of size_t to avoid implementation-defined type.
- Rename to bst_idx_t, to account for other types of indexing.
- Small cleanup to the base header.

src/collective/device_communicator_adapter.cuh

@@ -3,6 +3,8 @@
  */
 #pragma once
 
+#include <numeric>  // for accumulate
+
 #include "communicator.h"
 #include "device_communicator.cuh"