When a model has component folders (transformer, vae, etc.) but no model_index.json
at the root level, the loading would fail. This fix adds:

1. Base model fallback strategy:
   - Detect model type from model ID (ltx, wan, svd, cogvideo, mochi)
   - Load the known base model first
   - Then attempt to load fine-tuned components from the target model

2. Component detection and loading:
   - List files in the repo to find component folders
   - Load transformer, VAE components from the fine-tuned model
   - Fall back to base model if component loading fails

3. Better error messages:
   - Clear indication of what went wrong
   - Suggestions for alternative models

This fixes loading of models like Muinez/ltxvideo-2b-nsfw which have
all component folders but are missing the model_index.json file.