✨ Description

Third round of checkpoint improvements.

Functional changes:

Checkpoint format and model_type are merged into a single entry, with the original model_type becoming the format for external checkpoints. Temporary backward compatible.
Checkpoint conversion config has been updated to use the new checkpoint config format, i.e. input: CheckpointLoadConfig, output: CheckpointSaveConfig. Temporary backward compatible.

Dev changes:

Checkpoints are now modular, with all checkpointing formats following the same simple interface.
User can define arbitrary checkpoint format and mess with existing ones as needed. The default distributed and state dict format remain the official and supported format, but are no longer hard-coded.
Moved some content from FastLLMModel to MultiStage. FastLLMModel is basically gone, I'll probably merge the two classes soon.

🔍 Type of change

Select all that apply:

[ ] 🐛 Bug fix (non-breaking change that addresses a specific issue)
[x] 🚀 New feature (non-breaking change that adds functionality)
[x] ⚠️ Breaking change (a change that could affect existing functionality)
[ ] 📈 Performance improvement/optimization (improves speed, memory usage, or efficiency)
[x] 🛠️ Code refactor (non-functional changes that improve code readability, structure, etc.)
[ ] 📦 Dependency bump (updates dependencies, including Dockerfile or package changes)
[ ] 📝 Documentation change (updates documentation, including new content or typo fixes)
[ ] 🔧 Infrastructure/Build change (affects build process, CI/CD, or dependencies)