Oh, this looks like a bug using the inplace subtraction.
If you use this code, it seems b has the right result:
a = torch.from_numpy(a)
b = a - a[:, 7:8, :]
a -= a[:,7:8,:]
print(a)
print(b)

I was able to reproduce this issue in 1.0.0a0+dfad8b6 and 1.0.0.dev20181007.
Thanks for catching it! Could you create an issue in Gihhub 2?