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.

tests/cpp/plugin/test_sycl_predictor.cc

@@ -43,7 +43,7 @@ TEST(SyclPredictor, ExternalMemory) {
 }
 
 TEST(SyclPredictor, InplacePredict) {
-  bst_row_t constexpr kRows{128};
+  bst_idx_t constexpr kRows{128};
   bst_feature_t constexpr kCols{64};
   Context ctx;
   auto gen = RandomDataGenerator{kRows, kCols, 0.5}.Device(ctx.Device());
@@ -106,4 +106,4 @@ TEST(SyclPredictor, Multi) {
   TestVectorLeafPrediction(&ctx);
 }
 
-}  // namespace xgboost
+}  // namespace xgboost