Extend non_conservative to a 4-way string mode selector in ASE and TorchSim

[PicoCentauri]

Matches the non_conservative interface already implemented in the LAMMPS metatomic pair style (metatensor/lammps#47).

The non_conservative parameter was a bool in both MetatomicCalculator and MetatomicModel. It is now a string with four accepted values "off" (default), "on", "forces" and "stress".

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[frostedoyster]

Is this what we were talking about yesterday? I don't know if we need it because doing either conservatively (forces or stresses) has the same cost as doing both conservatively

I.e. C forces + NC stresses has the same cost as C forces + C stresses

Of course it's still useful if one wants to test the effects of turning one off, but it's such a niche curiosity that IMO one should hack it locally

[Luthaf]

My understanding is more that this is about allowing models that don't offer both outputs (for example because they were not trained with stress)

[Luthaf]

I think I'd prefer keeping backward compatibility by making this

Suggested change

	non_conservative="off",
	non_conservative:Union[bool, Literal["forces", "stress"]]=False,

[PicoCentauri]

I think this is making the API unclear if one allows "on" or True but to keep backward compatibility I agree we can change this.

[PicoCentauri]

And following #210 should we call it "force" or "forces"?

[Luthaf]

I think this is making the API unclear if one allows "on" or True

Yes, there should only be True, False, "force", "stress", no "on" here!

And following #210 should we call it "force" or "forces"?

I'd say "forces" is fine here, this is what you used in LAMMPS, right?

[PicoCentauri]

Okay I changed the synopsis.

Yes in LAMMPS we have <on/off/forces/stress>

[Luthaf]

Thanks!