Add mIoU metric
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Hmm the doctest failed again - maybe the full result needs to be on one single line cc @lhoestq for the final review Cool ! Feel free to merge if it's all good for you
This PR adds the mean Intersection-over-Union metric to the library, useful for tasks like semantic segmentation.

It is entirely based on mmseg's [implementation](https://github.com/open-mmlab/mmsegmentation/blob/master/mmseg/core/evaluation/metrics.py).

I've removed any PyTorch dependency, and rely on Numpy only.
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