create a grid of images and save it as a final image
    title : grid image name
    X : array of images

def visual(title, X, activation):
    '''create a grid of images and save it as a final image
    title : grid image name
    X : array of images
    '''
    assert len(X.shape) == 4

    X = X.transpose((0, 2, 3, 1))
    if activation == 'sigmoid':
        X = np.clip((X)*(255.0), 0, 255).astype(np.uint8)
    elif activation == 'tanh':
        X = np.clip((X+1.0)*(255.0/2.0), 0, 255).astype(np.uint8)
    n = np.ceil(np.sqrt(X.shape[0]))
    buff = np.zeros((int(n*X.shape[1]), int(n*X.shape[2]), int(X.shape[3])), dtype=np.uint8)
    for i, img in enumerate(X):
        fill_buf(buff, i, img, X.shape[1:3])
    cv2.imwrite('%s.jpg' % (title), buff)