Mistake in 1.2-automatic-differentiation.ipynb

[cgoecknerwald]

Text in notebook:

z.sum().backward()
print(x.grad)
tensor([[27., 27.],
[27., 27.]])

It says "You should have got a matrix of 4.5." This code actually creates

tensor([[27., 27.],
[27., 27.]])

It looks like it should instead be:

z.sum()
print(x.grad)