Adapt models to handle generic targets

[frostedoyster]

This PR defines the capabilities of each one of the current model in the documentation and performs the corresponding checks in the code. Part of #364.

Contributor (creator of pull-request) checklist

• Tests updated (for new features and bugfixes)?
• Documentation updated (for new features)?
• Issue referenced (for PRs that solve an issue)?

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📚 Documentation preview 📚: https://metatrain--386.org.readthedocs.build/en/386/

[DavideTisi]

There are only tests for saopbpnn where the functionality works, not for catching the errors in the other models, maybe add tests

[DavideTisi]

here a stupid question becouse i do not know the code. when atom is in target.layout.block(0).samples.names,
target.quantity can still be energy? if this is the case i can see a case where the error becomes:

"GAP only supports total-energy-like outputs, but a energy was provided"

which do not make much sense.
If this is the case split this error message for the case "atom" not in target.layout.block(0).samples.names.

[frostedoyster]

Yes, you're right. I will split the error message

[spozdn]

currently I see this:

raise ValueError(
"PET only supports total-energy-like outputs, "
f"but a {target.quantity} was provided"
)
should we instead propagate the request for generic targets to PET right away, given that PET does support 1) multidimensional targets, 2) targets per atom and per structure, and 3) both sum and mean aggregations?

[frostedoyster]

@spozdn The goal for this PR is to make sure that each type of target is handled by each architecture. We allowed spherical vector targets for SOAP-BPNN, but this is just to test the correctness of the whole infrastructure. Also GAP and the alchemical model could in principle fit a larger range of targets than those allowed here, as can PET, but that would be for a different PR. When it comes to PET, we're having a lot of integration issues even with the basic energy targets, so I wouldn't add more burden by adding more options until the current integration issues are solved and people want to use the extra features

[DavideTisi]

for me it is fine, for the discussion with PET I leave to you and sergey

[frostedoyster]

Thanks! Regarding the extra features for PET, I will make sure to open an issue. Waiting for #399 to be merged to avoid adding more problems to that PR