title: I can not save and restore a variable with placeholder in tensorflow
body: <p><strong><em>my codes is as follow:</em></strong></p>
  
  <pre><code>import tensorflow as tf
  import numpy as np
  def add_layer(input):
      v2 = tf.Variable(tf.random_normal([2, 2], dtype=tf.float32, name='v2'))
      tf.add_to_collection('h0_v2',v2)
      output=tf.matmul(input,v2)
      return output
  x1=tf.placeholder(tf.float32)
  outputs=add_layer(x)
  tf.add_to_collection('outputs', outputs)
  saver = tf.train.Saver()
  with tf.Session() as sess:
      sess.run(tf.global_variables_initializer())
      x1=np.random.random([2, 2])
      print(sess.run(outputs,feed_dict={x:x1}))
      save_path = saver.save(sess, './model.ckpt')
      print("model saved in file:", save_path)
  </code></pre>
  
  <p><strong>and then another codes is runned:</strong></p>
  
  <pre><code>import tensorflow as tf
  import numpy as np
  sess = tf.Session()
  saver = tf.train.import_meta_graph('./model.ckpt.meta')
  saver.restore(sess, tf.train.latest_checkpoint('./'))
  x2=np.random.random([2, 2])
  print(sess.run(tf.get_collection('outputs',feed_dict={x:x2})))
  print('model is loaded')
  sess.close()
  </code></pre>
  
  <p><strong>and then computer tells me that the 'x' is not defined, I do not know what is wrong.</strong></p>