Thank you ptrblck.
First of all , I needed to use softmax at last layer.
The reason I used F.relu at the last layer is because  when I calculate loss as below I get nan values after some epoch.
toutput = model(data)
loss = F.nll_loss(torch.log(output), target)

So, I used Relu at last layer and changes loss calculate as below:
eps = 1e-7
output = model(data)
loss = F.nll_loss(torch.log(output+eps), target)

If I remove Relu from last layer, how can I guarantee the term output in torch.log become non negative?