Update Marian config default vocabulary size
# What does this PR do?

Fixes #19296 

Looking at existing Marian models, their vocabulary size is set to pad token id + 1 ([example](https://huggingface.co/Helsinki-NLP/opus-mt-de-en/blob/main/config.json#L59)). This PR modifies the default vocabulary size such that a) it doesn't throw exceptions (must be > pad token id) and b) preserves this property.

Alternatively, the default for `decoder_start_token_id` and `pad_token_id` can be reduced 🙃 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Looks like a bugfix to me if the model cannot be instantiated. Looks good to me.