there may be a mistake

[fengxiaoshuai]

https://github.com/3dem/relion/blob/dfea290207df2ea63fee36f5edce34b8d5c7a1d0/src/ml_optimiser_mpi.cpp#L1205

cudaDeviceSynchronize() will only synchronize host with the currently set GPU, so the function just set the last device which be setted in MlDeviceBundle *b = new MlDeviceBundle(this);.