Objective function evaluation on GPU with minimal PCIe transfers (#2935)

* Added GPU objective function and no-copy interface.

- xgboost::HostDeviceVector<T> syncs automatically between host and device
- no-copy interfaces have been added
- default implementations just sync the data to host
  and call the implementations with std::vector
- GPU objective function, predictor, histogram updater process data
  directly on GPU

src/common/math.h

@@ -20,8 +20,8 @@ namespace common {
  * \param x input parameter
  * \return the transformed value.
  */
-inline float Sigmoid(float x) {
-  return 1.0f / (1.0f + std::exp(-x));
+XGBOOST_DEVICE inline float Sigmoid(float x) {
+  return 1.0f / (1.0f + expf(-x));
 }
 
 inline avx::Float8 Sigmoid(avx::Float8 x) {