py_nn.py

class Neural_Network(nn.Module):
    def __init__(self, ):
        super(Neural_Network, self).__init__()
        # parameters
        # TODO: parameters can be parameterized instead of declaring them here
        self.inputSize = 2
        self.outputSize = 1
        self.hiddenSize = 3
        
        # weights
        self.W1 = torch.randn(self.inputSize, self.hiddenSize) # 2 X 3 tensor
        self.W2 = torch.randn(self.hiddenSize, self.outputSize) # 3 X 1 tensor
        
    def forward(self, X):
        self.z = torch.matmul(X, self.W1) # 3 X 3 ".dot" does not broadcast in PyTorch
        self.z2 = self.sigmoid(self.z) # activation function
        self.z3 = torch.matmul(self.z2, self.W2)
        o = self.sigmoid(self.z3) # final activation function
        return o
        
    def sigmoid(self, s):
        return 1 / (1 + torch.exp(-s))
    
    def sigmoidPrime(self, s):
        # derivative of sigmoid
        return s * (1 - s)
    
    def backward(self, X, y, o):
        self.o_error = y - o # error in output
        self.o_delta = self.o_error * self.sigmoidPrime(o) # derivative of sig to error
        self.z2_error = torch.matmul(self.o_delta, torch.t(self.W2))
        self.z2_delta = self.z2_error * self.sigmoidPrime(self.z2)
        self.W1 += torch.matmul(torch.t(X), self.z2_delta)
        self.W2 += torch.matmul(torch.t(self.z2), self.o_delta)
        
    def train(self, X, y):
        # forward + backward pass for training
        o = self.forward(X)
        self.backward(X, y, o)
        
    def saveWeights(self, model):
        # we will use the PyTorch internal storage functions
        torch.save(model, "NN")
        # you can reload model with all the weights and so forth with:
        # torch.load("NN")
        
    def predict(self):
        print ("Predicted data based on trained weights: ")
        print ("Input (scaled): \n" + str(xPredicted))
        print ("Output: \n" + str(self.forward(xPredicted)))

I believe the function sigmoidPrime is defined wrong. it should be sigmoid(s)* (1-sigmoid(s)). Please correct me if I am wrong.

@Rahul-python
I admit the code can be improved and cleaned a bit. Your equation is right. But if you look closely, we calculated sigmoid in the forward pass, that's what the variable o represents. So we pass that o which becomes s in this particular sigoimdPrime function which is the sigmoid applied already. That's why it ends up being s * (1-s) as opposed to what you propose. Hopefully, it's a bit clearer now. Thanks for the feedback. I may work on improving this tutorial and provide more details and intuitions on design choices. :)