Thanks a lot!The code is
def train_epoch(model, loader, optimizer, epoch, n_epochs, print_freq=1):
batch_time = AverageMeter()
losses = AverageMeter()
error = AverageMeter()
# Model on train mode
model.train()

end = time.time()
input = []
target = []

for batch_idx,(input, target) in enumerate(loader):
    # Create vaiables
    if torch.cuda.is_available():
        input=input.cuda()
        target = target.cuda()
       output = model(input)
      **loss = torch.nn.functional.cross_entropy(output, target)**  This line is error.I want to know where is _thnn_nll_loss_forward.Thanks in advance.