[Fix] Mismatch Between bboxes and scores During NMS in Confusion Matrix Calculation

[mochiriku]

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Motivation

When running NMS within the calculate_confusion_matrix function, there is a bug where det_scores is not reduced even if det_bboxes is reduced.
The goal it to fix this bug.

Modification

Instead of overwriting det_bboxes with the output (concatenation of det_bboxes and det_scores) of the function nms, I modified it to store the output in dets once and then split it into det_bboxes and det_scores.
This allows det_scores to match det_bboxes.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMPreTrain.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[CLAassistant]

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