Add devices to FARMReader to manually select GPU, CPU etc

[tholor]

Is your feature request related to a problem? Please describe.
Currently, we only have the arg use_gpu to configure if the FARMReader should use a GPU. If this is set to true and there's no GPU, we'll fallback to running on a CPU. This is convenient for many dev scenarios, but in production setups there are cases where we need more granular control (e.g. which cuda device to run on, or throwing an error if no GPU is available)

Describe the solution you'd like
We can just use the same param devices as implemented here for DPR

CC @oryx1729