Factory pattern for model components

[dxqb]

This should make it easier to implement new models by having the model-specific code less spread around the entire codebase

includes #1210

[dxqb]

@O-J1 requested review from you because of possible implications of how modules are imported in this PR here:

OneTrainer/modules/util/create.py

Line 53 in c85e20e

factory.import_dir("modules/modelSampler", "modules.modelSampler")

ruff would remove regular imports because the modules aren't used anywhere - except that they register themselves when they are imported.

any downsides to importing just all python files that are in these directories?

[O-J1]

We might:

• Get some ruff false positives but I dont believe anything will be auto removed
• Import errors will still be fatal (same as current)
• Some impact to startup delays (same as current) because it all has to import and execute, even if it wont be used.

P.S I still think we should try to do conditional imports for the model actually neeed(?) havent ascertained if this would even be possible yet though