loss.py

import torch
import torch.nn as nn
import torch.optim as optim
from torch.autograd import Variable

class MyLoss(torch.nn.Module):
    def __init__(self):
        super(MyLoss, self).__init__()

    def forward(self, output, target):
        return torch.sum(output)

net = nn.Linear(5,5)
optimizer = optim.SGD(net.parameters(), lr=0.01)
criterion = MyLoss()

optimizer.zero_grad()
input = Variable(torch.randn(1,5))
pred = net(input)
loss = criterion(pred, None)
loss.backward()
print(net.weight.grad)