update Annotator.masks()

[Laughing-q]

• Annotator.masks() supports both gpu plotting(inference size) and cpu plotting(native size) now, also support both opencv and PIL.
• add a new argparse sign --retine-masks to use cpu plotting.
• add an additional arg img_gpu in Annotator.masks() and keep the initialization of im0.

I was trying to update the plot in val with Annotator.masks(), but it'll make a lot changes. Since the image in val is the big one composed with many small images, it's not easy to plot its masks as once. So I just keep it as before.