tensorflow2.0基本操作，二阶求导

import tensorflow as tf

w = tf.Variable(1.0)
b = tf.Variable(2.0)
x = tf.Variable(3.0)

with tf.GradientTape() as t1:
  with tf.GradientTape() as t2:
    y = x * w + b
  dy_dw, dy_db = t2.gradient(y, [w, b])
d2y_dw2 = t1.gradient(dy_dw, w)

print(dy_dw)
print(dy_db)
print(d2y_dw2)

assert dy_dw.numpy() == 3.0
assert d2y_dw2 is None