figure out better way to pass token type ids to model

[jonathanbratt]

currently, I attach tt_ids as an attribute to the tokenized input in tokenize_input. It feels like a misuse of attributes, but I also don't want to, say, pass the tt_ids as part of a list along with the tokenized input. We could just figure out the tt ids later, and calculate them when we need them to run the model?