Refactor system, freezer. Improve config structure. Add data postprocessing prior to criterion, metrics or logging.

[ibro45]

Description

• LighterSystem refactored. New config structure for datasets, schedulers, collate functions and metrics. Optimizers and schedulers config keys are now singular.
• Added data postprocessing for criterion, metrics, and logging.
• LighterFreezer is a callback now. Added exclude options to it.

Related Issue

• Make LighterFreezer a Callback #48
• LighterSystem's model's parameters start with model.. Unintuitive when using the freezer or adjusting the prefix of the checkpoint #75

Type of Change

• 📚 Examples / docs / tutorials / dependencies update
• 🔧 Bug fix (non-breaking change which fixes an issue)
• 🥂 Improvement (non-breaking change which improves an existing feature)
• 🚀 New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 🔐 Security fix

Checklist

• I've read the CODE_OF_CONDUCT.md document.
• I've read the CONTRIBUTING.md guide.
• I've updated the code style using make codestyle.
• I've written tests for all new methods and classes that I created.
• I've written the docstring in Google format for all the methods and classes that I used.