Fix loading transformer-only fine-tuned models (like Muinez/ltxvideo-2b-nsfw)

Some models on HuggingFace are not full pipelines but just fine-tuned components
(e.g., just the transformer weights). These have a config.json at root level with
_class_name pointing to a component class like 'LTXVideoTransformer3DModel'.

This fix adds:

1. Detection of component-only models:
   - Check for config.json at root level
   - Read _class_name to determine component type
   - Detect if it's a transformer, VAE, or other component

2. Proper loading strategy:
   - Load the base pipeline first (e.g., Lightricks/LTX-Video)
   - Then load the fine-tuned component from the model repo
   - Replace the base component with the fine-tuned one

3. Supported component classes:
   - LTXVideoTransformer3DModel → Lightricks/LTX-Video
   - AutoencoderKLLTXVideo → Lightricks/LTX-Video
   - UNet2DConditionModel, UNet3DConditionModel, AutoencoderKL

This allows loading models like Muinez/ltxvideo-2b-nsfw which are
fine-tuned transformer weights without a full pipeline structure.