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/tree/gpu_hist/gradient_based_sampler.cuh

@@ -25,6 +25,7 @@ class SamplingStrategy {
  public:
   /*! \brief Sample from a DMatrix based on the given gradient pairs. */
   virtual GradientBasedSample Sample(common::Span<GradientPair> gpair, DMatrix* dmat) = 0;
+  virtual ~SamplingStrategy() = default;
 };
 
 /*! \brief No sampling in in-memory mode. */