I am making a NN library without any other external NN lib and is implementing the Flatten layer. I know the forward implementation of flatten layer but is the backward just reshaping it or not? If yes is it I can just call a simple numpy reshape function to reshape it?
Yes, a simple reshape would do the trick. A flattening layer is just a tool for reshaping data/activations to make them compatible with other layers/functions. The flattening layer doesn't change the activations themselves, so there is no special backpropagation handling needed other than changing back the shape.