title: Sparse Tensor in tensorflow DataSets
body: <p>I have tensorflow program that work with TFRecord and i want to read the data with tf.contrib.data.TFRecordDataset but when i try to parse the example i get an exception: "TypeError: Failed to convert object of type  to Tensor"
  When trying with only</p>
  
  <p>The code is: </p>
  
  <pre><code>def _parse_function(example_proto):
      features = {"var_len_feature": tf.VarLenFeature(tf.float32),
                   "FixedLenFeature": tf.FixedLenFeature([10], tf.int64),
                   "label": tf.FixedLenFeature((), tf.int32default_value=0)}
  
  parsed_features = tf.parse_single_example(example_proto, features)
  return parsed_features["image"], parsed_features["label"]
  
  filenames = ["/var/data/file1.tfrecord", "/var/data/file2.tfrecord"]
  dataset = tf.contrib.data.TFRecordDataset(filenames)
  dataset = dataset.map(_parse_function)
  </code></pre>