If you reduced the number of classes to 6, your model should also output the logits for these 6 classes.
Currently it seems like your model has an output of shape [batch_size, 15].
I’m not sure to understand your labels properly, as I thought the indices point to the classes in the current sample, e.g. a tensor of [2, 5] would indicate class2 and class5 are present for the current sample, while all others are not.
Could you explain your labels a bit more, since I think I misunderstood them?