Keep NMS index gathering on cuda device

[Ghelfi]

Performs the unwrap of IoU mask directly on the cuda device in NMS.

This prevents device -> host -> device data transfer.

fixes #8713

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/8766

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[Ghelfi]

@NicolasHug for review