How can I calculate de AUC PR of my classifiers in a multiclass scenario?

I'm developing image classifiers in a context with 25k images and 50 classes. The dataset is imbalanced.

Some papers recommend AUC PR for comparing the performance of my classifiers in this setting. However, I was not able to find any implementation for calculating this metric for multiclass contexts. How can I calculate it?

If you have some piece of code that implements this, it would be very helpfull.