maint: `normalize_variants` as part of `pick_output`

[HaoZeke]

static torch::optional<std::string> normalize_variant(std::string variant_string)
{
    if (variant_string == "no" || variant_string.empty())
    {
        return torch::nullopt;
    }
    else
    {
        return variant_string;
    }
}

There's a version of this in LAMMPS, and now in GROMACS, perhaps it could just be moved into pick_output? ie.. https://docs.metatensor.org/metatomic/latest/torch/reference/cxx/misc.html#_CPPv4N15metatomic_torch11pick_outputENSt6stringEN5torch4DictINSt6stringE11ModelOutputEEN5torch8optionalINSt6stringEEE

[HaoZeke]

@PicoCentauri what do you think?

also cc @Luthaf

[PicoCentauri]

In general I think it is a very good idea.

The issue is a bit that the functions are slightly different in the engines as the input api is different. If you have a hood general approach go for it!

[Luthaf]

I'm not sure it is a good idea: we already take an optional value in this function, but mapping special strings to optional (like off in LAMMPS or no in GROMACS) is application specific IMO.