训练后，TensorFlow Hessian矩阵不会更新

我试图使用tf.hessians函数得到Hessian矩阵 . 在每次训练后，损失值和变量都会更新，而Hessian矩阵值保持不变 . 而且，它们不依赖于可以手动设置的初始变量值 . 实际上，我的问题类似于this one，但尚未得到答案 . 这是我用于测试的代码：

import tensorflow as tf

# Model parameters
W = tf.Variable([.3], dtype=tf.float32)
b = tf.Variable([-.3], dtype=tf.float32)

# Model input and output
x = tf.placeholder(tf.float32)
linear_model = W*x + b
y = tf.placeholder(tf.float32)

# loss
loss = tf.reduce_sum(tf.square(linear_model - y)) # sum of the squares

# optimizer
optimizer = tf.train.GradientDescentOptimizer(0.01)
train = optimizer.minimize(loss)

# training data
x_train = [1, 2, 3, 4]
y_train = [0, -1, -2, -3]

hess = tf.hessians(loss, [W, b])

# training loop
init = tf.global_variables_initializer()
sess = tf.Session()
sess.run(init) # reset values to wrong
for i in range(10):
    sess.run(train, {x: x_train, y: y_train})
    cur_hess, curr_W, curr_b, curr_loss = sess.run([hess, W, b, loss], {x: x_train, y: y_train})
    print("W: %s b: %s loss: %s"%(curr_W, curr_b, curr_loss))
    print('cur_hess', cur_hess)

以下是打印结果：

W: [-0.21999997] b: [-0.456] loss: 4.0181446
cur_hess [array([[60.]], dtype=float32), array([[8.]], dtype=float32)]
W: [-0.39679998] b: [-0.49552] loss: 1.8198745
cur_hess [array([[60.]], dtype=float32), array([[8.]], dtype=float32)]
W: [-0.459616] b: [-0.4965184] loss: 1.5448234
cur_hess [array([[60.]], dtype=float32), array([[8.]], dtype=float32)]
W: [-0.48454273] b: [-0.48487374] loss: 1.4825068
cur_hess [array([[60.]], dtype=float32), array([[8.]], dtype=float32)]
W: [-0.49684232] b: [-0.4691753] loss: 1.444397
cur_hess [array([[60.]], dtype=float32), array([[8.]], dtype=float32)]
W: [-0.5049019] b: [-0.45227283] loss: 1.409699
cur_hess [array([[60.]], dtype=float32), array([[8.]], dtype=float32)]
W: [-0.5115062] b: [-0.43511063] loss: 1.3761029
cur_hess [array([[60.]], dtype=float32), array([[8.]], dtype=float32)]
W: [-0.51758033] b: [-0.41800055] loss: 1.3433373
cur_hess [array([[60.]], dtype=float32), array([[8.]], dtype=float32)]
W: [-0.523432] b: [-0.40104443] loss: 1.3113549
cur_hess [array([[60.]], dtype=float32), array([[8.]], dtype=float32)]
W: [-0.52916396] b: [-0.38427448] loss: 1.2801344
cur_hess [array([[60.]], dtype=float32), array([[8.]], dtype=float32)]

所以，cur_hess没有更新，顺便说一下它只包含2个元素而不是4个 . 如何解决？此外，我尝试按照建议here两次应用tf.gradients，但是不会像tf.hessians那样更新值 . 同时，tf.gradients正确计算一阶导数，并在每次训练循环后更改它们 . 谢谢 .