Bug found in ConfusionMatrix.from_detections

[chiggins2024]

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Bug

Issue found in code when producing a confusion matrix for object detection. It seems like the FN was being added incorrectly to the matrix. Here is the code that was problematic for me. When removing the else condition, I was getting the correct TP value. It seems that num_classes, ends up being at the same position detection_classes[matched_detection_idx[j]]

        ```
for i, true_class_value in enumerate(true_classes):
            j = matched_true_idx == i
            print('sum(j)', sum(j))
            if matches.shape[0] > 0 and sum(j) == 1:
                result_matrix[
                    true_class_value, detection_classes[matched_detection_idx[j]]
                ] += 1  # TP
            else:
                result_matrix[true_class_value, num_classes] += 1  # FN

### Environment

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### Minimal Reproducible Example

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### Additional

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### Are you willing to submit a PR?

- [ ] Yes I'd like to help by submitting a PR!

[LinasKo]

Hi @chiggins2024 👋

Thank you for the report. We'll check it as soon as we can!
As we're transitioning away from the legacy MeanAveragePrecision and ConfusionMatrix, most likely the fix will come as a new ConfusionMatrix version.

[chiggins2024]

Hi thanks! do you know when we can anticipate this new ConfusionMatrix version to the ready ?

…

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