title: Shape must be rank 0 but is rank 1, parse_single_sequence_example
body: <p>For the past few days I have been having an issue with serializing data to tfrecord format and then subsequently deserializing it using parse_single_sequence example. I am attempting to retrieve data for use with a fairly standard RNN model, however this is my first attempt at using the tfrecords format and the associated pipeline that goes with it. </p>
  
  <p>Here is a toy example to reproduce the issue I am having:</p>
  
  <pre><code>import tensorflow as tf
  import tempfile
  from IPython import embed
  
  sequences = [[1, 2, 3], [4, 5, 1], [1, 2]]
  label_sequences = [[0, 1, 0], [1, 0, 0], [1, 1]]
  
  def make_example(sequence, labels):
  
      ex = tf.train.SequenceExample()
  
      sequence_length = len(sequence)
      ex.context.feature["length"].int64_list.value.append(sequence_length)
  
      fl_tokens = ex.feature_lists.feature_list["tokens"]
      fl_labels = ex.feature_lists.feature_list["labels"]
      for token, label in zip(sequence, labels):
          fl_tokens.feature.add().int64_list.value.append(token)
          fl_labels.feature.add().int64_list.value.append(label)
      return ex
  
  
  writer = tf.python_io.TFRecordWriter('./test.tfrecords')
  for sequence, label_sequence in zip(sequences, label_sequences):
      ex = make_example(sequence, label_sequence)
      writer.write(ex.SerializeToString())
  writer.close()
  
  tf.reset_default_graph()
  
  file_name_queue = tf.train.string_input_producer(['./test.tfrecords'], num_epochs=None)
  
  reader = tf.TFRecordReader()
  
  
  
  context_features = {
      "length": tf.FixedLenFeature([], dtype=tf.int64)
  }
  sequence_features = {
      "tokens": tf.FixedLenSequenceFeature([], dtype=tf.int64),
      "labels": tf.FixedLenSequenceFeature([], dtype=tf.int64)
  }
  
  ex = reader.read(file_name_queue)
  
  # Parse the example (returns a dictionary of tensors)
  context_parsed, sequence_parsed = tf.parse_single_sequence_example(
      serialized=ex,
      context_features=context_features,
      sequence_features=sequence_features
  )
  
  
  context = tf.contrib.learn.run_n(context_parsed, n=1, feed_dict=None)
  print(context[0])
  sequence = tf.contrib.learn.run_n(sequence_parsed, n=1, feed_dict=None)
  print(sequence[0])
  </code></pre>
  
  <p>The associated stack trace is:</p>
  
  <pre><code>Traceback (most recent call last):
  File "/usr/local/lib/python3.5/dist-packages/tensorflow/python/framework/common_shapes.py", line 594, in call_cpp_shape_fn
  status)
  File "/usr/lib/python3.5/contextlib.py", line 66, in exit
  next(self.gen)
  File "/usr/local/lib/python3.5/dist-packages/tensorflow/python/framework/errors.py", line 463, in raise_exception_on_not_ok_status
  pywrap_tensorflow.TF_GetCode(status))
  tensorflow.python.framework.errors.InvalidArgumentError: Shape must be rank 0 but is rank 1
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
  File "my_test.py", line 51, in 
  sequence_features=sequence_features
  File "/usr/local/lib/python3.5/dist-packages/tensorflow/python/ops/parsing_ops.py", line 640, in parse_single_sequence_example
  feature_list_dense_defaults, example_name, name)
  File "/usr/local/lib/python3.5/dist-packages/tensorflow/python/ops/parsing_ops.py", line 837, in _parse_single_sequence_example_raw
  name=name)
  File "/usr/local/lib/python3.5/dist-packages/tensorflow/python/ops/gen_parsing_ops.py", line 285, in _parse_single_sequence_example
  name=name)
  File "/usr/local/lib/python3.5/dist-packages/tensorflow/python/framework/op_def_library.py", line 749, in apply_op
  op_def=op_def)
  File "/usr/local/lib/python3.5/dist-packages/tensorflow/python/framework/ops.py", line 2382, in create_op
  set_shapes_for_outputs(ret)
  File "/usr/local/lib/python3.5/dist-packages/tensorflow/python/framework/ops.py", line 1783, in set_shapes_for_outputs
  shapes = shape_func(op)
  File "/usr/local/lib/python3.5/dist-packages/tensorflow/python/framework/common_shapes.py", line 596, in call_cpp_shape_fn
  raise ValueError(err.message)
  ValueError: Shape must be rank 0 but is rank 1
  </code></pre>
  
  <p>I posted this as a potential issue over on github though it seems I may just be using it incorrectly: <a href="https://github.com/tensorflow/tensorflow/issues/5225" rel="noreferrer">Tensorflow Github Issue</a>
  So with the background information out of the way, I'm just wondering if I am in fact making an error here? Any help in the right direction would be greatly appreciated, its been a few days and my poking around hasn't panned out. Thanks all! </p>