请问如下代码为什么报错：TypeError: Fetch argument None has invalid type <class 'NoneType'>

import tensorflow as tf

w1 = tf.Variable([[1, 2]])
w2 = tf.Variable([[3, 4]])

y = tf.matmul(w1, [[9], [10]])

grads = tf.gradients(y, [w1])

with tf.Session() as sess:
    sess.run(tf.global_variables_initializer())
    gradval = sess.run(grads)
    print(gradval)

tensorflow gradients好像不支持 int型的Tensor 的gradients 把w1的设置成float类型的例如tf.float32 gards就能算了

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