error in backward function - deep-learning

Question

I already defined a loss function in pytorch, but there is an error that I could not find solution. Here is my code:
<code>
class cust_loss(torch.nn.Module):
def __init__(self):
super(cust_loss, self).__init__()
def forward(self, input, target):
predicted_labels = torch.max(input, 1)[1]
minus = torch.max(input, 1)[1] - target
cust_distance = torch.sum(minus*minus).type(torch.FloatTensor)/predicted_labels.size()[0]
return cust_distance
######## within main function ######
criterion = cust_loss()#nn.CrossEntropyLoss()
Optimizer = optim.SGD(filter(lambda p: p.requires_grad, model_conv.parameters()), lr=1e-3, momentum=0.9)
loss = criterion(inputs, labels)
loss.backward()
Unfortunately, I got this error:
Traceback (most recent call last):
File "/home/morteza/PycharmProjects/transfer_learning/test_SkinDetection.py", line 250, in <module>
main(True)
File "/home/morteza/PycharmProjects/transfer_learning/test_SkinDetection.py", line 130, in main
loss.backward()
File "/home/morteza/anaconda3/lib/python3.6/site-packages/torch/autograd/variable.py", line 156, in backward
torch.autograd.backward(self, gradient, retain_graph, create_graph, retain_variables)
File "/home/morteza/anaconda3/lib/python3.6/site-packages/torch/autograd/__init__.py", line 98, in backward
variables, grad_variables, retain_graph)
File "/home/morteza/anaconda3/lib/python3.6/site-packages/torch/autograd/function.py", line 91, in apply
return self._forward_cls.backward(self, *args)
File "/home/morteza/anaconda3/lib/python3.6/site-packages/torch/autograd/_functions/basic_ops.py", line 38, in backward
return maybe_unexpand(grad_output, ctx.a_size), maybe_unexpand_or_view(grad_output.neg(), ctx.b_size), None
File "/home/morteza/anaconda3/lib/python3.6/site-packages/torch/autograd/variable.py", line 381, in neg
return Negate.apply(self)
File "/home/morteza/anaconda3/lib/python3.6/site-packages/torch/autograd/_functions/basic_ops.py", line 224, in forward
return i.neg()
AttributeError: 'torch.LongTensor' object has no attribute 'neg'
I could not solve it. I traced the code and compared it with a code that is error free, but I could not solve it. Moreover, I defined my inputs and labels as Variable with "requires_grad=True" parameter.
Please guide me how to solve it.
Thank you.