[ENH] refactor __init__ modules to no longer contain classes

[fkiraly]

This PR refactors the __init__ modules in models, moving any content such as neural network classes to submodules.

Imports will continue working unchanged, as the native contents of the module are still exported.

This is preparing a further structural refactors.

The moved files are also not changed, in order to retain the commit as one that moves files, hence retaining edit history. In order for this to take effect on main, PR #1739 must be merged first (even though this will result in an intermediate, non-passing state).