E0 bug in multi-head fine tuning

[bernstei]

There's an E0-related bug in multi-head-finetune if you have more theories in the E0s.json file than you actually use (and maybe in related circumstances as well)

Initiaklly I ran with "pbe" and "r2scan" in the E0s json, and added my own "hse_d3" key list with its own E0 values. However, when I looked at the fitting log, I saw that the Atomic Energies was a list with 3 sub-lists, each with E0s for one theory, but the label was apparently stripped, since it was a list, not a dict. Then I noticed that the initial force and stress errors were identical to before, but the energy error was much larger. When I removed the r2scan and only has pbe and hse_d3 (in that order) E0s, everything made sense. I suspect the way the relevant E0s are extracted from the json dict is not quite right.

[bernstei]

In general, it should be possible to clean up the E0s, e.g. remove keys that aren't there in the list of Zs, make sure that any Z that is present had a value specified, and, for multihead, fill in something (0?) for any Z that is missing in one theory but present in the other.

[gabor1]

Don't you think that putting in zeros for missing elements is bad? I'd rather not evaluate missing elements with that particular head at all! And if we don't evaluate, we shouldn't need to specify.

[bernstei]

Yes, we should make sure no Z from the data in a given head is missing a corresponding E0 entry. But the E0s dict is turned into a torch tensor and matrix-multiplied, so it has to have the same dimensions. I assume that the 0 entries then don't get used, but they need to be there.