[RAG] fix tokenizer
# What does this PR do?

- Introduce `as_target_tokenizer` context manager in `RagTokenizer` to later update the docs when `prepare_seq2seq_batch` is depricated.
- `RagTokenizer.prepare_seq2seq_batch` calls `super().prepare_seq2seq_batch`, but it does not inherit from `PreTrainedTokenizer`. Fix the method temporarily using the context manager.