So that the order in the `OrderedDict` is predictable, and thus the precise arrangements of the Theano expressions created is always the same, no matter what.

We've had a lot of trouble with flaky tests in Theano and pylearn2 caused by nondeterministic iteration orders, so unless we're absolutely sure that the resulting Theano object graph will be identical, it's best to prefer a deterministic order for everything (that's why we use OrderedDicts in most places that we do).