Fix RVC HuBERT subdirectory double-nesting issue
Technical fix:
- Add check to detect if .pt file is already in correct subdirectory
- Prevent creating nested subdirectories (e.g., chinese_hubert_base/chinese_hubert_base/)
- Use existing subdirectory path when model is already correctly placed
Fix RVC HuBERT model loading and compatibility
Technical changes:
- Fix Japanese HuBERT download URL (moved to public prj-beatrice repo)
- Fix content-vec-best model reference (was hubert-base-rvc)
- Add transformers HuBERT support with TransformersHubertWrapper
- Implement subdirectory structure for transformers models to avoid config conflicts
- Add auto-migration for existing models to new subdirectory structure
- Fix HubertConfig import scope issue in loaders.py
- Add config.json auto-download for transformers models
- Fix dtype conversion (float16/float32) in feature extraction
- Add dimension mismatch detection with helpful user messages
- Fix hubert_path parameter passthrough in conversion pipeline
- Keep SCNet as experimental due to audio buzzing artifacts
- Add clear warnings in CLI and UI tooltips about quality issues
- Implementation functionally complete with chunked processing
- Users warned about potential audio artifacts before use