Fix rotated box format conversion from XYXYXYXY to XYWHR

[AntoineSimoulin]

This PR fixes the rotated box format conversion from XYXYXYXY to XYWHR for both meta and ops directories. The conversion formula from #8841 was incorrect. We now compute $w=\sqrt{(x3-x1{)}^2+(y1-y3{)}^2}$ and $h=\sqrt{(x3-x2{)}^2+(y3-y2{)}^2}$.

We add tests to cover this conversion types. Please run the tests with:

pytest test/test_transforms_v2.py -vvv -k "TestConvertBoundingBoxFormat"
pytest test/test_ops.py -vvv -k TestBoxConvert

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/9019

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[NicolasHug]

Thank you @AntoineSimoulin !

[github-actions]

Hey @AntoineSimoulin!

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