Returns the actual image bytes for a given image.

    Args:
      run: The name of the run the image belongs to.
      tag: The name of the tag the images belongs to.
      index: The index of the image in the current reservoir.
      sample: The zero-indexed sample of the image to retrieve (for example,
        setting `sample` to `2` will fetch the third image sample at `step`).

    Returns:
      A bytestring of the raw image bytes.

def _get_individual_image(self, run, tag, index, sample):
    """
    Returns the actual image bytes for a given image.

    Args:
      run: The name of the run the image belongs to.
      tag: The name of the tag the images belongs to.
      index: The index of the image in the current reservoir.
      sample: The zero-indexed sample of the image to retrieve (for example,
        setting `sample` to `2` will fetch the third image sample at `step`).

    Returns:
      A bytestring of the raw image bytes.
    """
    if self._db_connection_provider:
      db = self._db_connection_provider()
      cursor = db.execute(
          '''
          SELECT data
          FROM TensorStrings
          WHERE
            /* Skip first 2 elements which are width and height. */
            idx = 2 + :sample
            AND tensor_rowid = (
              SELECT rowid
              FROM Tensors
              WHERE
                series = (
                   SELECT tag_id
                   FROM Runs
                   CROSS JOIN Tags USING (run_id)
                   WHERE
                     Runs.run_name = :run
                     AND Tags.tag_name = :tag)
                AND step IS NOT NULL
                AND dtype = :dtype
                /* Should be n-vector, n >= 3: [width, height, samples...] */
                AND (NOT INSTR(shape, ',') AND CAST (shape AS INT) >= 3)
              ORDER BY step
              LIMIT 1
              OFFSET :index)
          ''',
          {'run': run,
           'tag': tag,
           'sample': sample,
           'index': index,
           'dtype': tf.string.as_datatype_enum})
      (data,) = cursor.fetchone()
      return six.binary_type(data)

    events = self._filter_by_sample(self._multiplexer.Tensors(run, tag), sample)
    images = events[index].tensor_proto.string_val[2:]  # skip width, height
    return images[sample]