Consider whether `.generate()` should provide a default for `max_length`

[mattdangerw]

When you are generating up to an end of sequence id, as is often the case with seq2seq setups, you often want max_length to be the maximum possible context window provided by the model.

We should consider whether we should provide this as a default. That could create some simple usages, e.g. seq2seq_model.generate(french_sentence) -> english sentence, that could showcase our library well.