Change --gpu to --gpu_id to clarify the functionality

[vellamike]

please don't use underscore in longopts, it can get eaten by terminal borders and look like a space, longopts should be --like-this

[ntadimeti]

@vellamike thanks for the input! I'll change it accordingly

[ntadimeti]

@vellamike -- I saw your comment again. We don't have short option and long options. So, we actually access this argument with args.gpu_idx and a hiphen isn't compatible in that case. Am I making sense? Happy to discuss offline