PyTorch translation of Keras model is not learning

I am trying to translate an implementation of a Siamese neural network from Keras (tensorflow backend) to PyTorch.
This seemed to have worked quite smoothly but to my surprise the PyTorch model keeps an almost constant loss and doesn’t learn anything. I assume I am missing some PyTorch details.
I am trying to learn PyTorch but I think I am missing a crucial detail here which I don’t spot. The training loop and network principally works just the loss doesn’t reduce it stays more or less constant.
Maybe someone can help me with this?

My Keras network which works fine

def cosine_distance(vects):
    x, y = vects
    x = K.l2_normalize(x, axis=-1)
    y = K.l2_normalize(y, axis=-1)
    return -K.mean(x * y, axis=-1, keepdims=True)

def create_base_network(input_shape):
    '''Base network to be shared (eq. to feature extraction).
    input = Input(name='siamese_in', shape=input_shape)
    fwd_lstm = LSTM(512, name='fwdRNN', recurrent_dropout=.2) (input)
    bwd_lstm = LSTM(512, name='bwdRNN', recurrent_dropout=.2, go_backwards=True) (input)    
    x = Concatenate(name='rnn_concat')([fwd_lstm, bwd_lstm]) 
    model = Model(input, x)
    return model

base_network = create_base_network(input_shape)

input_a = Input(shape=input_shape)
input_b = Input(shape=input_shape)

# because we re-use the same instance `base_network`,
# the weights of the network
# will be shared across the two branches
vector_1 = base_network(input_a)
vector_2 = base_network(input_b)

x3 = Subtract()([vector_1, vector_2])
x3 = Multiply()([x3, x3])

x1_ = Multiply(name="square_vec1")([vector_1, vector_1])
x2_ = Multiply(name="square_vec2")([vector_2, vector_2])
x4 = Subtract(name="subtract")([x1_, x2_])
x5 = Lambda(cosine_distance, output_shape=eucl_dist_output_shape)([vector_1, vector_2])
conc = Concatenate(axis=-1)([x5, x4, x3])
x = Dense(512, activation="relu")(conc)
x = Dense(256, activation="relu")(x)
x = Dense(128, activation="relu")(x)
out = Dense(1, activation="sigmoid")(x)
model = Model([input_a, input_b], out)

model.compile(loss='mse', optimizer='adam')
for i in range(0, 30):
    batch_size=512[X_train_A, X_train_B], y_train,

My translated counter part looks like this:

class SiameseNetwork(nn.Module):
    def __init__(self, input_dim, rnn_hidden_dim):
        super(SiameseNetwork, self).__init__()
        self.rnn_hidden = rnn_hidden_dim
        self.rnn = nn.LSTM(input_dim, 
        self.cos = nn.CosineSimilarity(dim=1)
        self.out = nn.Sequential(
                    nn.Linear(rnn_hidden_dim*4 + 1, 512),
                    nn.Linear(512, 256),
                    nn.Linear(256, 128),
                    nn.Linear(128, 1),

    def forward_input_branch(self, x):
        hidden, last = self.rnn(x)
        h_fwd = last[0][0]
        h_bwd = last[1][0]
        return, h_bwd), -1)
    def transform_branch_output(self, x1, x2):
        # create representation as plain
        # substraction, substraction of squared representations
        # and cosine between representations
        # plain substract - squared
        x3 = x1 - x2
        x3 = x3 * x3
        # squared substract
        x4 = x1*x1 - x2*x2
        # cosine
        x5 = self.cos(x1, x2)
        x5 = x5.view(x5.size()[0], 1)
        # concat all
        concat =, x4, x3), -1)
        return concat

    def forward(self, input1, input2):
        x1 = self.forward_input_branch(input1)
        x2 = self.forward_input_branch(input2)
        combo = self.transform_branch_output(x1, x2)
        output = self.out(combo)
        return output
# put the net on the GPU
net = SiameseNetwork(len(selection), 512).cuda()

loss_fn = torch.nn.MSELoss(reduction='mean')
optimizer = optim.Adam(net.parameters())

for epoch in range(0, 20):
    for i, data in enumerate(train_dataloader):
        A, B, labels = data
        A, B, labels = A.cuda(), B.cuda(), labels.cuda()
        output = net(A, B)
        loss = loss_fn(output.squeeze(), labels)

    print("Epoch: {} - Loss {}\n".format(epoch, loss.item()))

Could you compare some hyperparameters, e.g. the learning rate between the two approaches?
Also, which criterion are you using in your Keras implementation.

I’m not completely sure, how the LSTM layers work in Keras, but it seems you are initializing two to create a bidirectional layer?

but it seems you are initializing two to create a bidirectional layer?

Yes, Keras has also a bidirectional combo layer but you can also set them up separately and set a property ‘backward’ on one of it so it flips the input within the layer.

I got the order of this triple wrong :slight_smile:
I zeroed the gradients too early.

Thanks anyway

This is actually the correct order.
Here is a code snippet given in Pytorch’s docs

output = net(input)
loss = criterion(output, target)

DId changing the order really fix it?

My mistake was that I provided the data, computed the loss and then zero-ed the grads, did back-prop and step()… If you place the ‘provide data’ call correctly, this is fine :).