Utility function to select the best device for a model

[frostedoyster]

This operation requires a considerable amount of lines and has many pitfalls (e.g. "cuda:2"). It needs to be replicated for every supported engine.

[Luthaf]

This would roughly correspond to https://github.com/metatensor/lammps/blob/4adedcb2575807f49042da6c267d70824db52295/src/ML-METATENSOR/pair_metatensor.cpp#L251-L330, extracted and made available to both C++ and Python