<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _get_lsun(directory, category, split_name):
 """Downloads all lsun files to directory unless they are there."""

  generator_utils.maybe_download(directory,
                                 _LSUN_DATA_FILENAME % (category, split_name),
                                 _LSUN_URL % (category, split_name))