test_requires_grad.py

import torch


x1 = torch.tensor(1.).requires_grad_(True)
x2 = torch.tensor(2.).requires_grad_(True)
w = torch.tensor(3.).requires_grad_(False)

y1 = w * x1
loss1 = torch.mean((y1-1)**2)
loss1.backward()

print(w.grad) # None

w.requires_grad_(True)
y2 = w * x2
loss2 = torch.mean((y2-1)**2)
loss2.backward()

print(w.grad) # 20