second_example.py

import tensorflow as tf

tf.enable_eager_execution()

X = tf.constant([[1., 2.], [3., 4.]])
y = tf.constant([[1.], [2.]])

w = tf.get_variable(name='w', shape=[2, 1], initializer=tf.constant_initializer([[1.], [2.]]))
b = tf.get_variable(name='b', shape=[1], initializer=tf.constant_initializer([[1.]]))

with tf.GradientTape() as tape:
    C = 0.5 * tf.reduce_sum(tf.square(tf.matmul(X, w) + b - y))
    
w_grad, b_grad = tape.gradient(C, [w, b])
    
print(C.numpy())
print(w_grad.numpy())
print(b_grad.numpy())