Allow HeAT to use the OpenCL backend

[Markus-Goetz]

Feature functionality Since torch 1.7 other backends than CPU and CUDA are supported. Among them is OpenCL. HeAT should be able to use it. Pay close attention whether torch's OpenCL backend supports both, CPUs and GPUs. If both, then there needs to be way to tell HeAT where to run the OpenCL code.

Additional context Requires torch 1.7