Allow None to be returned from hooks (fixes #1269)

[lantiga]

This is a fix for #1269.

Currently python_hook.cpp checks that individual values returned from a hook are tensors. However, as in the example with Conv without bias, they could be None. This fix simply allows None to be returned from a hook and updates the error message accordingly.

[lantiga]

To verify:

import torch
import torch.nn as nn
from torch.autograd import Variable

def hook(module, grad_input, grad_output):
    print(grad_input)
    return grad_input

a = Variable(torch.randn(1,1,3,3))
conv = nn.Conv2d(1, 1, (3,3), bias=False)
conv.register_backward_hook(hook)
b = conv(a)
b.backward()

[apaszke]

Actually already I have a fix for this in my branch. If someone is assigned to an issue then it means they're already working on it. Sorry!

[apaszke]

And thank you!

[lantiga]

Whoops, sorry for the noise.