in pydotprint, don't merge var with the same name. shared variable in update have distinct ellipse for their input and output in the graph. Both will have the same name with a different suffix.

scan documentation + scan taken outside of sandbox [ NOTE : gradient is not fully tested, so the version of scan that is outside of sandbox does not allow to compute gradients ]

When a gradient is zero because the variable has no influence on the result, now return a variable with same shape instead of a 0 scalar, so that we can rely on the shape being unchanged