The current code would raise NameError instead of the expected ValueError.

flake8 testing of https://github.com/PaddlePaddle/ERNIE on Python 3.8.0

$ __flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics__
```
./propeller/paddle/train/exporter.py:86:73: F821 undefined name 'v'
                    raise ValueError('unkonw inference spec type: %s' % v)
                                                                        ^
```