[ptbench.engine.device] Move AcceleratorProcessor to engine submodule, revamp it, and reflect changes in trainer

[ptbench.engine] Simplified, documented and created type hints for the ``callbacks`` module; Added type hints to the resource module

The CUDA_VISIBLE_DEVICES environement variable is set if a device is
specified or we raise an error if it has not been set.
Code for accelerator handling has been moved to its own utils script.