NonType Tensorflow

Question

Я попробовал модель Tensorflow, которая определяет k и x в уравнении прямой. Но я столкнулся с непонятной для меня проблемой. Я новичок в этом, поэтому я не могу объяснить вам мою проблему без тонны кода, извините. Ниже я оставлю свой код. Помоги мне, пожалуйста. Спасибо

import tensorflow as tf
import tensorflow.compat.v1 as tfc
graph = tf.Graph()
with graph.as_default():
    with tf.name_scope('placeholders'):
        x = tfc.placeholder(tf.float32, (3, 1))
        y = tfc.placeholder(tf.float32, (3, ))
    with tf.name_scope('weights'):
        W = tf.Variable(tfc.random_normal((1, 1)))
        b = tf.Variable(tfc.random_normal((1,)))
    with tf.name_scope('prediction'):
        y_pred = tf.matmul(x,W) + b
    with tf.name_scope('loss'):
        l = tf.reduce_sum((y-y_pred)**2)
    #Добавить оптимизацию тренировки
    with tf.name_scope('optim'):
        #Задать скорость заучивания .001, как рекомендовано выше.
        train_op = tfc.train.AdamOptimizer(.001).minimize(l)
    with tf.name_scope('summaries'):
        #Запись сводки о переменных(скалярных величинах) в заданный каталог журналов
        tf.summary.scalar('loss', l)
        #Объединение нескольких сводок в одну
        merged = tfc.summary.merge_all()

    train_writer = tfc.summary.FileWriter('/tmp/lr-train', graph)

    n_steps = 1000

    with tfc.Session() as sess:
        sess.run(tfc.global_variables_initializer())
        #Натренировать модель
        for i in range(n_steps):
            feed_dict = {x: [[1.], [2.], [3.]], y: [2., 3., 4.]}
            _, summary, loss = sess.run([train_op, merged, l], feed_dict=feed_dict)
            train_writer.add_summary(summary, i)

И у меня есть этот ответ

WARNING:tensorflow:From /Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/tensorflow_core/python/ops/resource_variable_ops.py:1635: calling BaseResourceVariable.__init__ (from tensorflow.python.ops.resource_variable_ops) with constraint is deprecated and will be removed in a future version.
Instructions for updating:
If using Keras pass *_constraint arguments to layers.
2020-01-09 22:31:46.911642: I tensorflow/core/platform/cpu_feature_guard.cc:142] Your CPU supports instructions that this TensorFlow binary was not compiled to use: AVX2 FMA
2020-01-09 22:31:46.929626: I tensorflow/compiler/xla/service/service.cc:168] XLA service 0x7fca2060fab0 initialized for platform Host (this does not guarantee that XLA will be used). Devices:
2020-01-09 22:31:46.929649: I tensorflow/compiler/xla/service/service.cc:176]   StreamExecutor device (0): Host, Default Version
Traceback (most recent call last):
  File "/Users/ss/Desktop/mac/TensorFlow/Z1.py", line 34, in <module>
    _, summary, loss = sess.run([train_op, merged, l], feed_dict=feed_dict)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/tensorflow_core/python/client/session.py", line 960, in run
    run_metadata_ptr)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/tensorflow_core/python/client/session.py", line 1168, in _run
    self._graph, fetches, feed_dict_tensor, feed_handles=feed_handles)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/tensorflow_core/python/client/session.py", line 477, in __init__
    self._fetch_mapper = _FetchMapper.for_fetch(fetches)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/tensorflow_core/python/client/session.py", line 266, in for_fetch
    return _ListFetchMapper(fetch)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/tensorflow_core/python/client/session.py", line 378, in __init__
    self._mappers = [_FetchMapper.for_fetch(fetch) for fetch in fetches]
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/tensorflow_core/python/client/session.py", line 378, in <listcomp>
    self._mappers = [_FetchMapper.for_fetch(fetch) for fetch in fetches]
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/tensorflow_core/python/client/session.py", line 263, in for_fetch
    (fetch, type(fetch)))
TypeError: Fetch argument None has invalid type <class 'NoneType'>