Mnist example gradient check

Hi, sorry newbie here, I’m trying to understand how to do gradient checking with PyTorch. I’m using the mnist example as a reference ( ). So first I wanted to get the input analytic gradients, which I think can be achieved by changing the following lines like this:

Line 81: data, target = Variable(data), Variable(target) --> data, target = Variable(data, requires_grad=True), Variable(target)
Insert into somewhere between lines 81-84: data.register_hook(print)

Then to get the numerical gradients I created a function like so:

def numerical_grad(input_, target, row_idx, col_idx):
    input_shp = input_.size()
    E = torch.zeros(input_shp)
    if args.cuda:
        E = E.cuda()
    eps = 0.001
    E[0][0][row_idx][col_idx] = eps
    E = Variable(E)
    M1 = input_ + E
    M2 = input_ - E
    out1 = model(M1)
    out2 = model(M2)
    l1 = F.nll_loss(out1, target)
    l2 = F.nll_loss(out2, target)
    grad = (l1 - l2)/(2*eps)
    return grad

I assumed this would give me the numerical gradient of the input at the specified row and column index, but it’s way way off. Am I doing something wrong? Thanks