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

反向傳播的時候，從當前層往上一層傳播的時候，自然是沒有 bias 的。