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

[dxqb]

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

I don't know if we should do that, but I can say that this PR doesn't change that.
all files are imported before and after this PR. the difference is that before, there is an explicit import for each file, and after this PR all files in modelSampler/*.py are imported

[dxqb]

has been tested as part of the Flux2 PR