title: Weighted sparse categorical cross entropy
body: <p>I am dealing with a semantic segmentation problem where the two classes in which I am interested (in addition to background) are quiet unbalanced in the image pixels. I am actually using sparse categorical cross entropy as a loss, due to the way in which training masks are encoded. Is there any version of it which takes into account class weights? I have not been able to find it, and not even the original source code of sparse_categorical_cross_entropy. I never explored the tf source code before, but the link to source code from API page doesn't seem to link to a real implementation of the loss function.</p>