feat: add checkpoint resume functionality, consolidate weights and custom weights path option for inference

[drishyakarki]

• Add ability to resume training from specific checkpoints
• Consolidate optimizer and model weights into single file
• Reduce number of checkpoint files to 2
• Add --weights argument to specify model weights path for inference
• Implement fallback logic to use best checkpoint from work_dir if no path specified
• Improve logging for model loading process
• Add file existence checks for weight paths
• Update help messages and documentation