def flatten_forward(x_input):
    """Perform the reshaping of the tensor of size `(K, L, M, N)`
        to the tensor of size `(K, L*M*N)`
    # Arguments
        x_input: np.array of size `(K, L, M, N)`
    # Output
        output: np.array of size `(K, L*M*N)`
    """
    K, L, M, N = x_input.shape

    output = x_input.reshape(K, L*M*N)
    return output