Fix bug in kv_store_t's insertion methods (#4444)

Update size_ field of kv_cuco_store_t with correct values.

Authors:
  - Naim (https://github.com/naimnv)

Approvers:
  - Chuck Hastings (https://github.com/ChuckHastings)
  - Seunghwa Kang (https://github.com/seunghwak)

URL: #4444

cpp/src/prims/kv_store.cuh

@@ -584,7 +584,7 @@ class kv_cuco_store_t {
         store_value_offsets.end(),
         kv_cuco_insert_and_increment_t<decltype(mutable_device_ref), KeyIterator>{
           mutable_device_ref, key_first, counter.data(), std::numeric_limits<size_t>::max()});
-      size_ += counter.value(stream);
+      size_ = counter.value(stream);
       resize_optional_dataframe_buffer<value_t>(store_values_, size_, stream);
       thrust::scatter_if(rmm::exec_policy(stream),
                          value_first,
@@ -636,7 +636,7 @@ class kv_cuco_store_t {
                                                   pred_op,
                                                   counter.data(),
                                                   std::numeric_limits<size_t>::max()});
-      size_ += counter.value(stream);
+      size_ = counter.value(stream);
       resize_optional_dataframe_buffer<value_t>(store_values_, size_, stream);
       thrust::scatter_if(rmm::exec_policy(stream),
                          value_first,
@@ -688,7 +688,7 @@ class kv_cuco_store_t {
         store_value_offsets.end(),
         kv_cuco_insert_and_increment_t<decltype(mutable_device_ref), KeyIterator>{
           mutable_device_ref, key_first, counter.data(), std::numeric_limits<size_t>::max()});
-      size_ += counter.value(stream);
+      size_ = counter.value(stream);
       resize_optional_dataframe_buffer<value_t>(store_values_, size_, stream);
       thrust::scatter_if(rmm::exec_policy(stream),
                          value_first,