Add snapshot_download import for model snapshot functionality

Refactor from_pretrained method for robust model loading and improved error handling

update Model definition

Update unified_cell_classifier.py

Update unified_cell_classifier.py

Update unified_cell_classifier.py

Update unified_cell_classifier.py

Update unified_cell_classifier.py

Update unified_cell_classifier.py

Update unified_cell_classifier.py

Aggiunta definizione classe UnifiedCellClassifier