Allow creating the documentation-only classes

[Luthaf]

Downstream code might create these at the top level, so erroring in the __init__ means that is it not even possible to import the package for documentation.

xref metatensor/metatrain#1144

@sofiia-chorna can you try building the docs with this PR?

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)?

Reviewer checklist

• CHANGELOG updated with public API or any other important changes?

[sofiia-chorna]

yes sure, checking

[sofiia-chorna]

seems ok! tox -e docs passed locally:
tox-docs-override.log

i did this in the pyproject.toml:

[tool.uv]
override-dependencies = [
    "metatomic-torch @ git+https://github.com/Luthaf/metatomic.git@create-doc-classes#subdirectory=python/metatomic_torch",
    "metatomic-ase @ git+https://github.com/Luthaf/metatomic.git@create-doc-classes#subdirectory=python/metatomic_ase",
]
``
architecture tests also passing

[Luthaf]

Ok, thanks for checking! I'll put this in another release then!