- A `DeviceOrd` struct is implemented to indicate the device. It will eventually replace the `gpu_id` parameter.
- The `predictor` parameter is removed.
- Fallback to `DMatrix` when `inplace_predict` is not available.
- The heuristic for choosing a predictor is only used during training.
* [R] Fix global feature importance.
* Add implementation for tree index. The parameter is not documented in C API since we
should work on porting the model slicing to R instead of supporting more use of tree
index.
* Fix the difference between "gain" and "total_gain".
* debug.
* Fix prediction.
