Differentiating through generator I

[gabrielarpino]

Error when differentiating through the following line:

Output is:

Fixed by changing to this:

Seems that it has trouble differentiating through generators where it has to infer the size of the matrix, like I for example. Worked if used eye(size(x)) instead.

[gabrielarpino]

Reproduce the error as follows:

using Nabla
f(x) = x.*I
x = [1, 2]
∇(f)(x)

[willtebbutt]

See #50 for discussion. Won't close this for now though.