fix unet script problem (#1092)
Summary:
when running `python run_sweep.py -m pytorch_unet -t eval -d cuda --jit`,
this would raise error `AttributeError: 'RecursiveScriptModule' object has no attribute 'n_classes'`
add a final mark to keep this attibute when script
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1092
Reviewed By: erichan1
Differential Revision: D39160269
Pulled By: xuzhao9
fbshipit-source-id: ef29321bac7d75835d22df9f0e4cf94b03d8f0c9
