split DSSM model got error

hi,dear 大佬
from the [issue](https://github.com/alibaba/EasyRec/issues/176) I tried the split script
but got the bug bellow:
```
python ../easy_rec/python/tools/split_model_pai.py --model_dir ../conf/dssm/_ckpt/export/final/ --user_model_dir ../conf/dssm/userModel/ --item_model_dir ../conf/dssm/itemModel/
Traceback (most recent call last):
File "../easy_rec/python/tools/split_model_pai.py", line 276, in
tf.app.run()
File "python3.7/site-packages/tensorflow_core/python/platform/app.py", line 40, in run
_run(main=main, argv=argv, flags_parser=_parse_flags_tolerate_undef)
File /python3.7/site-packages/absl/app.py", line 312, in run
_run_main(main, args)
File "/python3.7/site-packages/absl/app.py", line 258, in _run_main
sys.exit(main(argv))
File "../easy_rec/python/tools/split_model_pai.py", line 263, in main
part_dir=FLAGS.user_model_dir)
File "../easy_rec/python/tools/split_model_pai.py", line 203, in export
saver = tf_saver.Saver()
File "/python3.7/site-packages/tensorflow_core/python/training/saver.py", line 828, in __init__
self.build()
File "/python3.7/site-packages/tensorflow_core/python/training/saver.py", line 840, in build
self._build(self._filename, build_save=True, build_restore=True)
File "/python3.7/site-packages/tensorflow_core/python/training/saver.py", line 865, in _build
raise ValueError("No variables to save")
ValueError: No variables to save

```
could u pls help me ?
thx

Start by reproducing the command from the issue and inspect easy_rec/python/tools/split_model_pai.py around export at line 203 and main at line 263. Trace why the TensorFlow Saver sees no variables during the split. Done means the command completes and produces the requested user and item model directories.

Written by the indexing model from the issue text.