updating T5 and BART models to support Prefix Tuning
# What does this PR do?
1. updating T5 and BART models to support Prefix Tuning. Currently, passing `past_key_value` fails. This PR fixes it. Doesn't impact any current functionality. 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Hey, just for reference could you provide a link to an issue or something explaining what `prefix tuning` is?