Builds tf.train.Features from (string -> int/float/str list) dictionary.

def _dict_to_tf_features(example_dict):
  """Builds tf.train.Features from (string -> int/float/str list) dictionary."""
  features = {k: _item_to_tf_feature(v, k) for k, v
              in six.iteritems(example_dict)}
  return tf.train.Features(feature=features)