How the original image is displayed in the visualization.
Values:
OVERLAY_TYPE_UNSPECIFIED (0):
Default value. This is the same as NONE.
NONE (1):
No overlay.
ORIGINAL (2):
The attributions are shown on top of the
original image.
GRAYSCALE (3):
The attributions are shown on top of
grayscaled version of the original image.
MASK_BLACK (4):
The attributions are used as a mask to reveal
predictive parts of the image and hide the
un-predictive parts.