Fix vertical_flip_bounding_boxes wrong corner reorder for XYXYXYXY format

[Zenka737]

Summary

vertical_flip_bounding_boxes used the same corner reordering index as
horizontal_flip_bounding_boxes for the XYXYXYXY format — a copy-paste
error that produces corners in the wrong order after a vertical flip.

Root Cause

XYXYXYXY stores four corners as [TL, TR, BR, BL] (indices 0–7, pairs of xy).
After flipping all y-coordinates:

• Horizontal flip swaps TL↔TR and BL↔BR → reorder [2,3, 0,1, 6,7, 4,5] ✓
• Vertical flip swaps TL↔BL and TR↔BR → reorder must be [6,7, 4,5, 2,3, 0,1]

The vertical-flip path was incorrectly reusing the horizontal-flip index
[2, 3, 0, 1, 6, 7, 4, 5], returning corners as [old-TR, old-TL, old-BL, old-BR]
instead of the correct [new-TL, new-TR, new-BR, new-BL].

Fix

# Before (wrong — copy-pasted from horizontal_flip)
bounding_boxes = bounding_boxes[:, [2, 3, 0, 1, 6, 7, 4, 5]]

# After (correct for vertical flip)
bounding_boxes = bounding_boxes[:, [6, 7, 4, 5, 2, 3, 0, 1]]

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/9556

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