Add confusion matrix

[osanseviero]

This PR adds the confusion matrix metric. Some flake errors forced me to change two small type checks.

[lvwerra]

LGTM! 🚀

[lvwerra]

Actually looks like the doctests are failing. Did you test if the docstring example result match reality?

[osanseviero]

It should be good now :)
Edit: test passing locally but not here, will look into it next week

[osanseviero]

Sorry for all the noise with the commits. After some testing, I could pinpoint the issue of different string representations in Windows vs Ubuntu. E.g.

Ubuntu

{'confusion_matrix': array([[1, 0, 1],\n       [0, 2, 0],\n       [1, 1, 0]])}\n

Windows

{'confusion_matrix': array([[1, 0, 1],\n       [0, 2, 0],\n       [1, 1, 0]], dtype=int64)}\n

An alternative would be to just print the shape or sum of the confusion matrix, but this loses a bit of the point. This is also a bit confusing because other metrics such as mean_iou or mse don't face this issue.

[osanseviero]

Ok - the solution was to use ellipsis so it works with or without dtype. Let me know if this is ok. I also changed the Actions workflow so that if the ubuntu test fails, the windows one keeps running. This will consume more resources per workflow run, so let me know if I should revert that part - it would have been useful to identify this bug much faster.

[lvwerra]

Thanks for fixing it. Looks all good!