In this lecture, the professor says that one problem with the sigmoid function is that its outputs aren't zero-centered. Are the explanation provided by the professor regarding why this is bad is that the gradient of our loss w.r.t. the weights $\frac{\partial L}{\partial w}$ which is equal to $\frac{\partial L}{\partial \sigma}\frac{\partial \sigma}{\partial w}$ will always be either negative or positive and we'll have a problem updating our weights as she shows in this slide, we won't be able to move in the direction of the vector $(1,-1)$. I don't understand why since she only talks about one component of our gradient and not the whole vector. if the components of the gradient of our loss will have different signs which will allow us to adjust to different directions I'm I wrong? But the thing that I don't understand is how this property generalizes to non zero-centered functions and non-zero centered data?