Hi everyone,
so, I am trying to implement an Autoencoder for text based on LSTMs. It’s the foundation for something more sophisticated.
However, it always learns to output 4 characters which rarely change during training and for the rest of the string the output is the same on every index.
I use a one hot encoding.
Training:
...
        criterion = nn.BCEWithLogitsLoss()
        losses = []
        optimizer = optim.Adam(self.parameters(), lr)
        list = util.get_all_targets()
        random.shuffle(list)
        for i, item in enumerate(list):
            target_tensor = util.example_to_tensor(item)
            input_tensor = util.example_to_tensor(item)
            if is_cuda:
                target_tensor = target_tensor.cuda()
                input_tensor = input_tensor.cuda()
            input_var =  Variable(input_tensor)
            target_var = Variable(target_tensor)
            output = self(input_var, len(item))
            loss = 0
            optimizer.zero_grad()
            for i, item in enumerate(output[0]):
                loss += criterion(item, target_var[0][i])
            loss.backward()
            optimizer.step()

The net:

class DecoderRNN(nn.Module):
    def __init__(self, hidden_size, output_size, num_layers, input_size = hidden_size):
        super(DecoderRNN, self).__init__()
        self.hidden_size = hidden_size
        self.num_layers = num_layers
        self.linear = nn.Linear(input_size, hidden_size)
        self.lstm = nn.LSTM(hidden_size, output_size, num_layers, batch_first=True)
        self.sigmoid = nn.Sigmoid()

    def forward(self, encoded_input):
        decoded_output, hidden = self.lstm(encoded_input)
        decoded_output = self.sigmoid(decoded_output)
        return decoded_output

class EncoderRNN(nn.Module):
    def __init__(self, input_size, hidden_size, num_layers, output_size = hidden_size, dropout = 0):
        super(EncoderRNN, self).__init__()
        self.hidden_size = hidden_size
        self.num_layers = num_layers

        self.lstm = nn.LSTM(util.get_letters_num(), hidden_size, num_layers, dropout=dropout, batch_first=True)
        self.relu = nn.ReLU()

    def forward(self, x):
        x = self.lstm(x)
        x = x[0]  # Dont care about hidden states
        return self.relu(x)

class LSTMAutoEncoder(nn.Module):
    def __init__(self, input_size, hidden_size, num_layers):
        super(LSTMAutoEncoder, self).__init__()
        self.encoder = EncoderRNN(input_size, hidden_size, num_layers)
        self.decoder = DecoderRNN(hidden_size, input_size, num_layers)
        if is_cuda:
            self.encoder = self.encoder.cuda()
            self.decoder = self.decoder.cuda()

    def forward(self, input, length):
        input = self.encoder(input)
        decoded_output = self.decoder(input) #Softmax doesnt change a thing here, same problem
        return decoded_output

The data is formatted as:
(Batch, CharIndex, OneHotForThatChar)
With Batch always being 1.
I understand, that I’ll be having some error in thinking, but I just can’t figure it out.
Thanks for your help!
PS: The Net is taken and modified from LSTM autoencoder architecture 199
Edit: Tried more things and it seems, the first two letters will get correct after a few epochs. Rest is still always the same letter.