TF - update (vision_)encoder_decoder past variable
# What does this PR do?

I missed these 2 in the `past` refactor PR (https://github.com/huggingface/transformers/pull/15944). It reverts the error from as mentioned here (https://github.com/huggingface/transformers/pull/16230) to its previous form, as tracked here (https://github.com/huggingface/transformers/issues/15983).
_The documentation is not available anymore as the PR was closed or merged._