Added cuda checks to enable inference via the universal demo

[Jrokisky]

Hi all,
I added some conditionals around the cuda calls to enable some inference via cpu.
The changes made allowed me to successfully run:

python -u mseg_semantic/tool/universal_demo.py   --config=mseg_semantic/config/test/default_config_360_ss.yaml   model_name mseg-3m   model_path ./mseg-3m-480p.pth input_file $IMAGE

[johnwlambert]

thanks very much for this contribution @Jrokisky , I've been meaning to add the cpu inference support. I'm traveling the next few days but will double check the integration tests when I get back and try to get this merged in by early next week

[Jrokisky]

@johnwlambert Sounds good. Thanks for sharing your work in such an easy to use way!