Lazy initialization of device vector. (#5173)

* Lazy initialization of device vector.

* Fix #5162.

* Disable copy constructor of HostDeviceVector.  Prevents implicit copying.

* Fix CPU build.

* Bring back move assignment operator.

src/predictor/gpu_predictor.cu

@@ -339,6 +339,7 @@ class GPUPredictor : public xgboost::Predictor {
       // the first step only modifies prediction store in learner without following code.
       InitOutPredictions(cache_emtry->second.data->Info(),
                          &(cache_emtry->second.predictions), model);
+      CHECK_EQ(cache_emtry->second.predictions.Size(), out_preds->Size());
       cache_emtry->second.predictions.Copy(*out_preds);
     }
   }