Implement GK sketching on GPU. (#5846)

* Implement GK sketching on GPU.
* Strong tests on quantile building.
* Handle sparse dataset by binary searching the column index.
* Hypothesis test on dask.

tests/cpp/helpers.h

@@ -168,7 +168,9 @@ class SimpleRealUniformDistribution {
   ResultT operator()(GeneratorT* rng) const {
     ResultT tmp = GenerateCanonical<std::numeric_limits<ResultT>::digits,
                                     GeneratorT>(rng);
-    return (tmp * (upper_ - lower_)) + lower_;
+    auto ret = (tmp * (upper_ - lower_)) + lower_;
+    // Correct floating point error.
+    return std::max(ret, lower_);
   }
 };