Gradient based sampling for GPU Hist (#5093)

* Implement gradient based sampling for GPU Hist tree method.
* Add samplers and handle compacted page in GPU Hist.

include/xgboost/base.h

@@ -193,6 +193,36 @@ class GradientPairInternal {
     return g;
   }
 
+  XGBOOST_DEVICE GradientPairInternal<T> &operator*=(float multiplier) {
+    grad_ *= multiplier;
+    hess_ *= multiplier;
+    return *this;
+  }
+
+  XGBOOST_DEVICE GradientPairInternal<T> operator*(float multiplier) const {
+    GradientPairInternal<T> g;
+    g.grad_ = grad_ * multiplier;
+    g.hess_ = hess_ * multiplier;
+    return g;
+  }
+
+  XGBOOST_DEVICE GradientPairInternal<T> &operator/=(float divisor) {
+    grad_ /= divisor;
+    hess_ /= divisor;
+    return *this;
+  }
+
+  XGBOOST_DEVICE GradientPairInternal<T> operator/(float divisor) const {
+    GradientPairInternal<T> g;
+    g.grad_ = grad_ / divisor;
+    g.hess_ = hess_ / divisor;
+    return g;
+  }
+
+  XGBOOST_DEVICE bool operator==(const GradientPairInternal<T> &rhs) const {
+    return grad_ == rhs.grad_ && hess_ == rhs.hess_;
+  }
+
   XGBOOST_DEVICE explicit GradientPairInternal(int value) {
     *this = GradientPairInternal<T>(static_cast<float>(value),
                                   static_cast<float>(value));