Implement unified update prediction cache for (gpu_)hist. (#6860)

* Implement utilites for linalg.
* Unify the update prediction cache functions.
* Implement update prediction cache for multi-class gpu hist.

tests/cpp/tree/test_gpu_hist.cu

@@ -390,7 +390,10 @@ void UpdateTree(HostDeviceVector<GradientPair>* gpair, DMatrix* dmat,
   hist_maker.Configure(args, &generic_param);
 
   hist_maker.Update(gpair, dmat, {tree});
-  hist_maker.UpdatePredictionCache(dmat, preds);
+  hist_maker.UpdatePredictionCache(
+      dmat,
+      VectorView<float>{
+          MatrixView<float>(preds, {preds->Size(), 1}, preds->DeviceIdx()), 0});
 }
 
 TEST(GpuHist, UniformSampling) {