* Multi-GPU support in GPUPredictor. - GPUPredictor is multi-GPU - removed DeviceMatrix, as it has been made obsolete by using HostDeviceVector in DMatrix * Replaced pointers with spans in GPUPredictor. * Added a multi-GPU predictor test. * Fix multi-gpu test. * Fix n_rows < n_gpus. * Reinitialize shards when GPUSet is changed. * Tests range of data. * Remove commented code. * Remove commented code.