It may be that my description is not detailed enough to cause your misunderstanding. when followed the repository(https://github.com/e-lab/ENet-training 6) to train my own dataset, i met a error. And i find that this  error  occurs in  train.lua(err = loss:forward(y,yt) – updateOutput).
Here’s the error shown:
libraries/torch/extra/cunn/lib/THCUNN/ClassNLLCriterion.cu:52: void cunn_ClassNLLCriterion_updateOutput_kernel(Dtype *, Dtype *, Dtype *, long *, Dtype *, int, int, int, int) [with Dtype = float, Acctype = float]: block: [0,0,0], thread: [1,0,0] Assertion t> = 0 && t <n_classes failed