Use flag library validation support to mark --model as required instead of abusing assertions.
Use flag library validation support to mark --model as required instead of abusing assertions.

Assertions shouldn't be used for validating public interfaces, see: http://google.github.io/styleguide/pyguide.html#244-decision

This also gives a more succinct and clear error message, without mentioning assertions nor a distracting traceback.

PiperOrigin-RevId: 271369362

tensorflow/lingvo,tensorflow/lingvo,tensorflow/lingvo,tensorflow/lingvo