Pytorch的runtime error

#===========PyTorch: Autograd==============
#====A PyTorch Variable is a node in a computational graph===
#==============x.data is a Tensor
#=============x.grad is a Variable of gradients(same shape as x.data)==
#========x.grad.data is a Tensor of gradients===========================
import torch 
from torch.autograd import Variable
N,D_in, H,D_out = 64, 1000, 100, 10
x = Variable(torch.randn(N, D_in), requires_grad=False)
y = Variable(torch.randn(N, D_out), requires_grad=False)
w1 = Variable(torch.randn(D_in, H), requires_grad=True)
w2 = Variable(torch.randn(H,D_out), requires_grad=True)

learning_rate = 1e-6
for t in range(500):
  y_pred = x.mm(w1).clamp(min=0).mm(w2)
  loss = ((y_pred-y).pow(2).sum())
  #should be if w1.grad is not None:
  if w1.grad:
    w1.grad.data.zero_()
  if w2.grad: 
    w2.grad.data.zero_()
  loss.backward()
  
  w1.data -= learning_rate * w1.grad.data
  w2.data -= learning_rate * w2.grad.data
  print(loss)