Support non-PBC FairChem models

[ryanliu30]

Currently, the FairChem model requires PBC. Since people also use FairChem to train on non-PBC systems, it is valuable to include non-PBC as well.

[orionarcher]

Thanks for pointing this out! We will look into this, tagging @AdeeshKolluru.

[AdeeshKolluru]

Thanks for pointing it out, looks like it was inadvertently added. I'll change that soon!