Simplified checkpoint loading

[jlamypoirier]

✨ Description

Refactored checkpoint loading to make is simpler. The code is still a mess, but will be more manageable for future work.

Functional changes:

• Distributed checkpoint loading will determine automatically whether to use the fast (same format) or safe (different format) loading scheme. This means checkpoints will load correctly after a config change in mid-training, and in some case pretrained checkpoint loading will be faster.
• Added some safety checks in checkpoint configs.

🔍 Type of change

Select all that apply:

• 🐛 Bug fix (non-breaking change that addresses a specific issue)
• 🚀 New feature (non-breaking change that adds functionality)
• ⚠️ Breaking change (a change that could affect existing functionality)
• 📈 Performance improvement/optimization (improves speed, memory usage, or efficiency)
• 🛠️ 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)