Extracts 'tarinfo' from 'tgz' and writes to 'dst_path'.

def _extract_file(self, tgz, tarinfo, dst_path, buffer_size=10<<20):
    """Extracts 'tarinfo' from 'tgz' and writes to 'dst_path'."""
    src = tgz.extractfile(tarinfo)
    dst = tf_v1.gfile.GFile(dst_path, "wb")
    while 1:
      buf = src.read(buffer_size)
      if not buf:
        break
      dst.write(buf)
      self._log_progress(len(buf))
    dst.close()
    src.close()