MarianTokenizer: get vocab length in `as_target_tokenizer` mode
# What does this PR do?

When using the MarianTokenizer with a different source and target SPMs, it is expected to get the length of the target vocabulary when performing:
```python
with tokenizer.as_target_tokenizer():
    print(len(tokenizer))
```

This PR introduces a minimal change to get the correct size depending on the selected SPM.


## Before submitting
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Did you write any new necessary tests?

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15943). All of your documentation changes will be reflected on that endpoint.