Correct truncation for RoBERTa in 2-input GLUE
Extend the truncation fix to the two-input case.

(Example: currently throws if running MRPC with `max_seq_length=32`.)
Looks good to me, thanks!<|||||>Actually, for single-sentence inputs, do we expect one or two terminating `</s>`s? Currently we will generate two, I think.<|||||>@LysandreJik we can now update the GLUE scripts to use the newly added option `add_special_tokens` (added to all the tokenizers), don't you think?<|||||>Indeed, we should use it. I'll add that soon.