temptative fix for the case where a calculator does not return anything

[liam-o-marsh]

simple python test:

import pyscf, rascaline
mol = rascaline.systems.wrap_system(pyscf.M(atom='H 0 0 -10; H 0 0 10'))
rascaline.calculators.NeighborList(5.0,False).compute(mol)

Since the cutoff is smaller than the distance between hydrogens, there are no pairs found by compute_neighbors, causing assert_ne!(keys.count(), 0); to panic (at rascaline/src/calculator.rs:59:9).

Those three lines prevent the panic by making Calculator::prepare() return an empty TensorMap early.

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📚 Documentation preview 📚: https://rascaline--308.org.readthedocs.build/en/308/

[Luthaf]

Thanks you for the PR! This should be added as a test somewhere, either in Python or directly in the Rust code.

I'm not sure if an early return is the best solution here, since it might interact weirdly with key selections or samples/properties selection when giving predefined values. It think that returning an empty vec instead of the failing assert if there are no keys would be a nicer solution. I'll think of a test for this (slightly convoluted) case

[liam-o-marsh]

https://github.com/Luthaf/rascaline/actions/runs/9319582029/job/25654465095?pr=308 the fact that it failed like this on only one platform tastes like cosmic rays? anywho, yeah that way makes more sense

[Luthaf]

The CI error does seems unrelated, not sure why it is failing though. @PicoCentauri did you seen anything like this while writing the spliner code?

[Luthaf]

Ok, CI error should be fixed now. Can you rebase this branch on top of master?

[liam-o-marsh]

you're welcome!