Do not change the output from tuple to list - to match PT's version
# What does this PR do?

Current `TFPegasusDecoder` change `all_self_attns` and `all_cross_attns` from `tuple` to `list` before return the outputs:

https://github.com/huggingface/transformers/blob/3c4fbc616f74120c3900d07c772b7d2d9c7a53dd/src/transformers/models/pegasus/modeling_tf_pegasus.py#L1061-L1065

Pytorch `PegasusDecoder` returns them as tuple (i.e. not extra step to change them to `list`). This causes (WIP) PT-TF-equivalence test failed. 

This PR fixes this, as well as for 5 other models (+ template) with the same issue:

-  [x] bart
-  [x] blenderbot
-  [x] blenderbot_small
-  [x] marian
-  [x] mbart

Think change `list` to `tuple` is acceptable (i.e. not a [too] breaking change)? cc @sgugger for this aspect.

TF: @gante @Rocketknight1 
Bart, Pegasus: @patrickvonplaten




The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15918). All of your documentation changes will be reflected on that endpoint.<|||||>I can't think of any general TF reason to prefer lists to tuples, so if the tests pass and nothing broke then LGTM!<|||||>Would like to hear from @patrickvonplaten when he has time , since he is more involved in these models.<|||||>Nice!