Allow the HIP backend

[Markus-Goetz]

Feature functionality Since torch 1.7 other backends than CPU and CUDA are supported. Among them is the HIP, aka AMD's CUDA, backend. HeAT should be able to use it. Moreover, when using the 'gpu' device in HeAT, the framework should automatically determine whether CUDA or HIP is meant.

Additional context Requires torch 1.7