The sequence length has nothing to do with the number of neurons. The size is all set here:
self.gru = nn.GRU(input_size = embedding_size, hidden_size = hidden_size, batch_first = True, num_layers=stacked_layers)

Your figure shows an “unrolled” RNN, but this is just for visualization behind the recursive nature behind RNNs. Each blue box labeled LSTM is the exact same network.