Updating decode_gif to default to DISPOSE_DO_NOT to fix animated gif loading

[sg3-141-592]

There are errors loading animated gifs where the frame disposal method is set to DISPOSAL_UNSPECIFIED #9009

This is because when an animated gif has disposal mode DISPOSAL_UNSPECIFIED, decode_gif is defaulting to "background" drawing mode. It needs to be DISPOSE_DO_NOT instead where changes are overwritten onto the previous canvas. This is the default behaviour of web browsers and libraries like Pillow - GifImagePlugin.py.

After this the example gif subsequent frames display correctly

import cv2
import torchvision

gif = torchvision.io.read_image('jake.gif').numpy().transpose(0, 2, 3, 1)
image = cv2.cvtColor(gif[5], cv2.COLOR_RGB2BGR)
cv2.imshow(f'Frame 5', image)
cv2.waitKey()

[pytorch-bot]

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[AntoineSimoulin]

@sg3-141-592 thanks for putting this fix together! This is looking good. I only adjusted the linting. Feel free to fill the Contributor License Agreement and submit the PR for review!

[sg3-141-592]

@sg3-141-592 thanks for putting this fix together! This is looking good. I only adjusted the linting. Feel free to fill the Contributor License Agreement and submit the PR for review!

Sure CLA check should clear in the next hour

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