RuntimeError: Trying to backward through the graph a second time

backward_error_example.py

import torch
import numpy as np

# This will be share by both iterations and will make the second backward fail !
a = torch.ones(3, requires_grad=True) * 4

# Instead, do
# a = torch.tensor(np.ones(3) * 4, requires_grad=True)

for i in range(10):
    d = torch.sum(a)
    # The first here will work but the second will not !
    d.backward()