Make image predictor thread-safe

[adamp87]

• Problem: SAM2ImagePredictor is not thread-safe as it stores state variables after set_image() or set_image_batch(). This can be cumbersome to handle in a multi-threaded environment e.g. Web App.
• Solution: return object by set_image(), which holds "state" variables. Pass returned object to predicts(). This makes predictor stateless.
• Backward compatibility: store last computed object of set_image() in SAM2ImagePredictor