Add ignore_index to Jaccard loss

[simonreise]

All the other segmentation losses have ignore_index parameter, and JaccardLoss does not. The ignore_index logic was copied from Dice Loss

[qubvel]

Thanks!

[codecov]

Codecov Report

Attention: Patch coverage is 37.50000% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
segmentation_models_pytorch/losses/jaccard.py	37.50%	10 Missing ⚠️

Files with missing lines	Coverage Δ
segmentation_models_pytorch/losses/jaccard.py	73.01% <37.50%> (-12.70%)	⬇️

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