Deterministic GPU histogram. (#5361)

* Use pre-rounding based method to obtain reproducible floating point
  summation.
* GPU Hist for regression and classification are bit-by-bit reproducible.
* Add doc.
* Switch to thrust reduce for `node_sum_gradient`.

src/data/ellpack_page_source.cc

@@ -29,14 +29,14 @@ bool EllpackPageSource::Next() {
 EllpackPage& EllpackPageSource::Value() {
   LOG(FATAL) << "Internal Error: "
                 "XGBoost is not compiled with CUDA but EllpackPageSource is required";
-  EllpackPage* page;
+  EllpackPage* page { nullptr };
   return *page;
 }
 
 const EllpackPage& EllpackPageSource::Value() const {
   LOG(FATAL) << "Internal Error: "
                 "XGBoost is not compiled with CUDA but EllpackPageSource is required";
-  EllpackPage* page;
+  EllpackPage* page { nullptr };
   return *page;
 }