title: How to feed input string with shape=[?] to tensorflow model using golang
body: <p>Python code of train model:</p>
  
  <pre><code>input_schema = dataset_schema.from_feature_spec({
      REVIEW_COLUMN: tf.FixedLenFeature(shape=[], dtype=tf.string),
      LABEL_COLUMN: tf.FixedLenFeature(shape=[], dtype=tf.int64)
  })
  </code></pre>
  
  <p>In python predictions work fine. Example of client:</p>
  
  <pre><code>loaded_model = tf.saved_model.loader.load(sess, ["serve"], '/tmp/model/export/Servo/1506084916')
  input_dict, output_dict =_signature_def_to_tensors(loaded_model.signature_def['default_input_alternative:None'])
  start = datetime.datetime.now()
  out = sess.run(output_dict, feed_dict={input_dict["inputs"]: ("I went and saw this movie last night",)})
  print(out)
  print("Time all: ", datetime.datetime.now() - start)
  </code></pre>
  
  <p>But golang client does not work:</p>
  
  <pre><code>m, err := tf.LoadSavedModel("/tmp/model/export/Servo/1506084916", []string{"serve"}, &amp;tf.SessionOptions{})
  if err != nil {
      panic(fmt.Errorf("load model: %s", err))
  }
  
  data := "I went and saw this movie last night"
  t, err := tf.NewTensor([]string{data})
  if err != nil {
      panic(fmt.Errorf("tensor err: %s", err))
  }
  fmt.Printf("tensor: %v", t.Shape())
  
  output, err = m.Session.Run(
      map[tf.Output]*tf.Tensor{
          m.Graph.Operation("save_1/StringJoin/inputs_1").Output(0): t,
      }, []tf.Output{
          m.Graph.Operation("linear/binary_logistic_head/predictions/classes").Output(0),
      }, nil,
  )
  if err != nil {
      panic(fmt.Errorf("run model: %s", err))
  }
  </code></pre>
  
  <p>I got error:</p>
  
  <blockquote>
    <p>panic: run model: You must feed a value for placeholder tensor
    'Placeholder' with dtype string and shape [?]
             [[Node: Placeholder = Placeholder_output_shapes=[[?]], dtype=DT_STRING, shape=[?],
    _device="/job:localhost/replica:0/task:0/cpu:0"]]</p>
  </blockquote>
  
  <p>How can I present <code>shape=[?]</code> tensor with golang? Or I need to change input format of python training script?</p>
  
  <p>UPD:</p>
  
  <p>This string <code>"save_1/StringJoin/inputs_1"</code> I received after running this python-code:</p>
  
  <pre><code>for n in sess.graph.as_graph_def().node:
      if "inputs" in n.name:
          print(n.name)
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>transform/transform/inputs/review/Placeholder 
  transform/transform/inputs/review/Identity 
  transform/transform/inputs/label/Placeholder 
  transform/transform/inputs/label/Identity 
  transform/transform_1/inputs/review/Placeholder 
  transform/transform_1/inputs/review/Identity 
  transform/transform_1/inputs/label/Placeholder 
  transform/transform_1/inputs/label/Identity 
  save_1/StringJoin/inputs_1 
  save_2/StringJoin/inputs_1
  </code></pre>