Step 3 of Backpropagation the size of z2 and Theta2'*delta3 are not equal.

https://www.coursera.org/learn/machine-learning/programming/AiHgN/neural-network-learning/discussions/threads/EObmU4LkEeWOJwp8z3qGpQ

反向传播的时候，从当前层往上一层传播的时候，自然是没有 bias 的。