tensorflow编码错误：TypeError: unsupported operand type(s) for *: ‘float‘ and ‘NoneType‘

tensorflow 编码出现错误：TypeError: unsupported operand type(s) for *: ‘float’ and ‘NoneType’

原码如下：

import tensorflow as tf
w=tf.Variable(tf.constant(5,dtype=float)) #给w赋随机值，初始值为float 5
lr=0.2 #学习率设为0.2
epoch=20 #设置循环迭代次数
for i in range(epoch):   #for循环20次对l