But here comes another problem
I run the following code, the first iter works well,  but for the second iter  there comes out an error showing that the parameter.grad is a None type which means the backward failed.

"""The tirst iter"""

loss = ((input*parameter).abs()-1).abs().sum()
loss.backward()
parameter = parameter - (0.01 * Variable(parameter.grad.data, requires_grad=True, volatile=False))

"""The second iter """
loss = ((input*parameter).abs()-1).abs().sum()
loss.backward()
parameter = parameter - (0.01 * Variable(parameter.grad.data, requires_grad=True, volatile=False))


Any idea about this?
Thank you~