--master-gpu has no effect

https://github.com/vlimant/mpi_learn/blob/master/mpi_learn/mpi/manager.py#L29
in

`def get_device(comm, num_masters=1, gpu_limit=-1, gpu_for_master=False):`

gpu_for_master is not used anymore, and I think it is hard to know at that point whether the global rank is a "master" or not. 
We might need to change the location of the device discovery closer to where models are created