Feature/50 panoptic quality

[justusschock]

New PR based on original PR from @niberger #929

What does this PR do?

Add the panoptic quality metric, as defined in the reference paper.
This implementation differ from the reference implementation of the COCO dataset on the following points:

• The categories have to be declared at metric initialization.
• Any undefined category will be considered as an unlabelled (void) region.
• The input format for panoptic images are Tensor of shape (height, width, 2), where the first number of each pixel is the category id and the second number is the instance id.
• No support for group of instances (crowds in COCO).

Fixes #50

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

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[justusschock]

NOTE before squash merging, make sure that @niberger is mentioned as co-author

[Borda]

@justusschock can we rather finish #929 seems still open still up to date...

[justusschock]

@Borda yes, this was an intermediate work because I couldn't push to the other branch