import os in glue_models.py

[lgessler]

It is needed by GlueModel.save

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[lgessler]

@googlebot I signed it!

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[jameswex]

Thanks @lgessler for finding the issue and providing a fix. Looks like this was due to an error in our process of importing the code to GitHub. We'll fix our import script to ensure we don't have this regression again.