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/gbm/test_gbtree.cc

@@ -171,7 +171,7 @@ TEST(GBTree, ChoosePredictor) {
 }
 
 TEST(GBTree, ChooseTreeMethod) {
-  bst_row_t n_samples{128};
+  bst_idx_t n_samples{128};
   bst_feature_t n_features{64};
   auto Xy = RandomDataGenerator{n_samples, n_features, 0.5f}.GenerateDMatrix(true);
 

tests/cpp/gbm/test_gbtree.cu

@@ -18,7 +18,7 @@
 namespace xgboost {
 void TestInplaceFallback(Context const* ctx) {
   // prepare data
-  bst_row_t n_samples{1024};
+  bst_idx_t n_samples{1024};
   bst_feature_t n_features{32};
   HostDeviceVector<float> X_storage;
   // use a different device than the learner