Do xWy

    Parameters
    ----------
    x : NDArray
        (input_size x seq_len) x batch_size
    W : NDArray
        (num_outputs x ny) x nx
    y : NDArray
        (input_size x seq_len) x batch_size
    input_size : int
        input dimension
    seq_len : int
        sequence length
    batch_size : int
        batch size
    num_outputs : int
        number of outputs
    bias_x : bool
        whether concat bias vector to input x
    bias_y : bool
        whether concat bias vector to input y

    Returns
    -------
    output : NDArray
        [seq_len_y x seq_len_x if output_size == 1 else seq_len_y x num_outputs x seq_len_x] x batch_size

def bilinear(x, W, y, input_size, seq_len, batch_size, num_outputs=1, bias_x=False, bias_y=False):
    """Do xWy

    Parameters
    ----------
    x : NDArray
        (input_size x seq_len) x batch_size
    W : NDArray
        (num_outputs x ny) x nx
    y : NDArray
        (input_size x seq_len) x batch_size
    input_size : int
        input dimension
    seq_len : int
        sequence length
    batch_size : int
        batch size
    num_outputs : int
        number of outputs
    bias_x : bool
        whether concat bias vector to input x
    bias_y : bool
        whether concat bias vector to input y

    Returns
    -------
    output : NDArray
        [seq_len_y x seq_len_x if output_size == 1 else seq_len_y x num_outputs x seq_len_x] x batch_size
    """
    if bias_x:
        x = nd.concat(x, nd.ones((1, seq_len, batch_size)), dim=0)
    if bias_y:
        y = nd.concat(y, nd.ones((1, seq_len, batch_size)), dim=0)

    nx, ny = input_size + bias_x, input_size + bias_y
    # W: (num_outputs x ny) x nx
    lin = nd.dot(W, x)
    if num_outputs > 1:
        lin = reshape_fortran(lin, (ny, num_outputs * seq_len, batch_size))
    y = y.transpose([2, 1, 0])  # May cause performance issues
    lin = lin.transpose([2, 1, 0])
    blin = nd.batch_dot(lin, y, transpose_b=True)
    blin = blin.transpose([2, 1, 0])
    if num_outputs > 1:
        blin = reshape_fortran(blin, (seq_len, num_outputs, seq_len, batch_size))
    return blin