Use DART tree weights when computing SHAPs (#5050)

This PR fixes tree weights in dart being ignored when computing contributions.

* Fix ellpack page source link.
* Add tree weights to compute contribution.

src/data/ellpack_page.cc

@@ -10,6 +10,8 @@ namespace xgboost {
 
 class EllpackPageImpl {};
 
+EllpackPage::EllpackPage() = default;
+
 EllpackPage::EllpackPage(DMatrix* dmat, const BatchParam& param) {
   LOG(FATAL) << "Internal Error: XGBoost is not compiled with CUDA but EllpackPage is required";
 }