def gram_matrix(y):
    """ Returns the gram matrix of y (used to compute style loss) """
    (b, c, h, w) = y.size()
    features = y.view(b, c, w * h)
    features_t = features.transpose(1, 2)   #C和w*h转置
    gram = features.bmm(features_t) / (c * h * w)   #bmm 将features与features_t相乘
    return gram