Suspicious memory explosions - Add __del__ method to differentiable

[JasperSnoek]

Something strange is happening when the graph is dynamically altered that seems to prevent nodes (or graphs) from being garbage collected.

Adding a del method to differentiable that removes links to a node from its parents might help solve this.

[kswersky]

This can be done using Python's weakref module. Essentially, you can modify it so that forward links from the parent to its children are weak references that get destroyed when the children have no more strong references to them. You have to be a bit careful to make sure that you don't wrap a weakref in an object like a list that acts as a strong reference, but it should be very possible to solve this problem. I will submit a pull request soon with a fix.