https://discuss.pytorch.org/t/mysterious-behaviour-of-cross-entropy-loss/107285

cuda_nll_loss2d.py

import torch
import torch.nn.functional as F

is_cuda = True

input = torch.randn(1, 19, 32, 64, requires_grad=True)
target = torch.randint(22, size=(1,32,64,))
print(input.dim())

print("number of out-of-bound targets", (target > 18).sum())

if is_cuda:
    input = input.cuda()
    target = target.cuda()

out = F.cross_entropy(input, target, weight=None, ignore_index=255, reduction='none')
print(out)