some feedback

[Joachim Nolten]

This had me stumped for a second:

errors = layer.back_propagate(errors)

with

@neurons.zip(errors).map { |neuron, error|
      neuron.back_propagate(error) }.sum

I had to remember, that back_propagate returns a Vector, so calling sum on an array of Vectors will be a Vector addition resulting in another Vector.

I think in general the object oriented style makes it easier to follow along, but also easier to miss some of the finer points. The course sounds interesting!