How to use the numjs.concatenate function in numjs

var forward_sequence = [];
        var reverse_sequence = [];
        var i;
        var s;
        var N = sequence.length;
        for (i = 0; i < N; i++) {
            s = [sequence[i][0] / scale_factor, sequence[i][1] / scale_factor, sequence[i][2], sequence[i][3], sequence[i][4]];
            forward_sequence.push(s);
        }
        for (i = N - 1; i >= 0; i--) {
            s = [forward_sequence[i][0], forward_sequence[i][1], forward_sequence[i][2], forward_sequence[i][3], forward_sequence[i][4]];
            reverse_sequence.push(s);
        }
        var output_fw = enc_fw_lstm.encode(forward_sequence);
        var output_bw = enc_bw_lstm.encode(reverse_sequence);
        var output = nj.concatenate([output_fw, output_bw]);
        var mu = nj.add(nj.dot(output, enc_mu_w), enc_mu_b);
        // optimization:
        if (temp > 0) {
            var presig = nj.add(nj.dot(output, enc_sigma_w), enc_sigma_b);
            var sigma = nj.sqrt(nj.exp(presig));
            var eps = nj.multiply(nj.array(random_normal_vector(), 'float32'), temp);
            var z = nj.add(mu, nj.multiply(eps, sigma));
        } else {
            var z = mu;
        }
        return z.tolist();
    };

var z = nj.array(y);

        var init_state = nj.tanh(nj.add(nj.dot(z, enc_w), enc_b));

        var c = init_state.slice([0, dec_num_units]).clone();
        var h = init_state.slice([dec_num_units, 2 * dec_num_units]).clone();

        var rnn_state;
        var dx, dy, pen_down, pen_up, pen_end;
        var pdf;
        var x = nj.array([0, 0, 0, 0, 0]);
        var result = [];
        var lstm_input;

        for (var i = 0; i < max_seq_len; i++) {
            lstm_input = nj.concatenate([x, z]);
            rnn_state = dec_lstm.forward(lstm_input, h, c);
            pdf = get_pdf(rnn_state);
            [dx, dy, pen_down, pen_up, pen_end] = sample(pdf, temp, softmax_temp);
            result.push([dx, dy, pen_down, pen_up, pen_end]);
            if (pen_end === 1) {
                return result;
            }
            x = nj.array([dx / scale_factor, dy / scale_factor, pen_down, pen_up, pen_end]);
            h = rnn_state[0];
            c = rnn_state[1];
        }
        result.push([0, 0, 0, 0, 1]);
        return result;

    };

LSTMCell.prototype.forward = function(x, h, c) {
    var concat = nj.concatenate([x, h]);
    var hidden = nj.add(nj.dot(concat, this.Wfull), this.bias);
    var num_units = this.num_units;
    var forget_bias = this.forget_bias;

    var i = nj.sigmoid(hidden.slice([0 * num_units, 1 * num_units]));
    var g = nj.tanh(hidden.slice([1 * num_units, 2 * num_units]));
    var f = nj.sigmoid(nj.add(hidden.slice([2 * num_units, 3 * num_units]), forget_bias));
    var o = nj.sigmoid(hidden.slice([3 * num_units, 4 * num_units]));

    var new_c = nj.add(nj.multiply(c, f), nj.multiply(g, i));
    var new_h = nj.multiply(nj.tanh(new_c), o);

    return [new_h, new_c];
};
LSTMCell.prototype.encode = function(sequence) {

function LSTMCell(num_units, input_size, Wxh, Whh, bias) {
    this.num_units = num_units;
    this.input_size = input_size;
    this.Wxh = Wxh;
    this.Whh = Whh;
    this.bias = bias;
    this.forget_bias = 1.0;
    this.Wfull = nj.concatenate([Wxh.T, Whh.T]).T;
}
LSTMCell.prototype.zero_state = function() {