add device argument to Processor class

[erceguder]

Hey, I've basically added a device argument for the Processor class constructor to allow for cuda, although defaults to cpu. Directly built upon this PR. Backward compatible.

[patrickvonplaten]

Hmm is this really necessary? Couldn't we just do the device conversion after loading?

[erceguder]

Surely we can do as such. Yet, this might be useful when the processor_id argument passed to the Processor class constructor is a variable, and we want to utilize cuda for checkpoint-based processors. In this case, the proposed change avoids the need for explicitly type-checking whether the processor needs a checkpoint, to load it on cuda.