import torch
x = torch.autograd.Variable(torch.Tensor([2]),requires_grad=True)
y = 5*x**2
y.backward()
x.grad
we get 20 because 5x^2 -> 10x = 10 * 2 = 20
And I would like to know the code inside backward() ,  there might be some function return 10 * input , right?