[BREAKING] Remove higher level model imports

[CompRhys]

Summary

Attempt to explore if lazy imports can help with the metatensor import issues.

Import errors being suppressed by the try-except-pass caused a misleading error message about MetatensorModel being unimportable. see #178

[janosh]

given that these changes, don't surface better error messages on failed imports and it looks a bit over-engineered, i suggest we remove the imports in torch_sim/models/__init__.py and just leave the file blank

[orionarcher]

I am fine removing the imports, can we put [breaking] in the PR title so I can track it in the next changelog?

[janosh]

@orionarcher the recommended way to highlight breaking changes in github's auto-generated release notes is to have a release.yml file like this one https://github.com/janosh/pymatviz/blob/main/.github/release.yml and assign the breaking label to this PR (no harm having it in the PR title as well ofc except you should remove it in the auto-generated release notes to avoid redundancy). based on the labels, you then get these kinds of release notes with a single click from github https://github.com/janosh/pymatviz/releases/tag/v0.15.0