TF: final bias as a layer in seq2seq models (replicate TFMarian fix)
# What does this PR do?

This PR replicates the exact same change as in https://github.com/huggingface/transformers/pull/18833 (applied to TFMarian) to the other seq2seq TF models. **_The change is exactly the same for all models._**

In essence, weights that are not in layers are not stored/loaded with `.save_weights()` and `.load_weights()`, the functions we use to store to/load from the hub. These changes move `final_logits_bias` to a layer. Many models do NOT use this bias, but some do.

⚠️ Prior to this change, existing TF models from `Helsinki-NLP` (`TFMarian`) were wrong (and new conversions failed the automatic checks). I will revisit the canonical models using these architectures to ensure they are okay, and open PRs with weights if not.
_The documentation is not available anymore as the PR was closed or merged._