Development liquids

[juliayang]

Summary

Fix assertion error in stress array size when FLARE_Atom loads a training structure with stress = np.array(None) (since training was not done with stress).

Checklist

Code is in the standard Python style.

• Run Black on your local machine.
• All existing tests pass.

Note that the CI system will run all the above checks. But it will be much more
efficient if you already fix most errors prior to submitting the PR.