tokenization_marian.py: use current_spm for decoding
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Fixes #10294 


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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
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Hi @patil-suraj! 
Thanks for your review. As you suggested, I started updating the code and docs where `decode` or `batch_decode` is used.
Doing so, I noticed RAG model also has the same issue: in `decode` and `batch_decode`, `generator` is used instead of `current_tokenizer`. Do you want me to also update that model and its docs accordingly in this PR? <|||||>Hey, I submitted my changes and also fixed the RAG tokenizer. Please let me know if I missed something or you want me to change any of the fixes. I can rebase and force-push again. <|||||>> do that inside the context manager, so we should also update all the ex

@patil-suraj @sgugger, wouldn't it be nicer to just do `as_target_tokenizer` in the `batch_decode` and `decode` function itself? Because decoding usually corresponds to the "target tokenizer" -> I think this would be nicer for the user<|||||>We could do that, but the reported issue is about not being able to decode the source tokens as the `decode` always uses `spm_target`.  So if we do `as_target_tokenizer` inside `decode` then source tokens will be decoded using `spm_target`, which will cause the same issue.<|||||>True, yeah I was a bit off there!

Ok, I understand the fix now. It's a very problematic fix however because it's a big backward breaking change. In 99% of the cases people use `batch_decode` for the target outputs and we don't really want people to update their code just so that the source targets work correctly I think...If I understand correctly this PR would change the default behavior of `batch_decode(...)` which is a no-go sadly...

Could we maybe somehow let the `current_spm` default to `target_spm` when using `batch_decode`, `decode` so that we don't have any breaking changes & then add maybe a new context manager `as_source_tokenizer`? or just add a optional arg to `decode` for Marian?

cc @LysandreJik @sgugger <|||||>It's not very complicated to add the `as_source_tokenizer` context manager. Another solution is to add a flag `use_source_tokenizer` (defaults to False) to `decode` and `batch_decode`.

In any case, backward-compatibility is paramount so it needs to be fully enforced.<|||||>`use_source_tokenizer` seems a better option to me, since the tokenizer already behaves like a source tokenizer by default so adding `as_source_tokenizer` seems a bit confusing IMO.

@Mehrad0711 , here's how we could now implement this
1. `current_spm` should always default to `source_spm` except inside the  `as_target_tokenizer`.
2. As Sylvain suggested, add the `use_source_tokenizer` argument to `decode` and `batch_decode`, if it's `True` use `source_spm` in `convert_tokens_to_string`. 
3. `convert_tokens_to_string` should never use `current_spm` as it defaults to `source_spm` and this would break backward-compatibility.

And the user should now pass `use_source_tokenizer` to decode source tokens<|||||>Thanks. I can proceed with the suggested implementation.
 However, passing `use_source_tokenizer` to `convert_tokens_to_string` requires updating `PreTrainedTokenizer`'s `_decode` method. Since `use_source_tokenizer` is passed to `convert_tokens_to_string` for all tokenizers, afaiu, the tokenizer classes for all models that implement their own `convert_tokens_to_string` should be updated individually to accept `use_source_tokenizer` (even though it's not used for some such as encoder-only models). 
I think a potential workaround can be adding instance checks within `_decode` to see if the tokenizer accepts that argument, or perhaps use function overloading.
Please let me know how you want me to proceed.<|||||>I think we can work around this by setting an internal attribute with the value of `use_source_tokenizer` passed by the user. This way we can recover it in `convert_tokens_to_string` without having to overload any other methods. What do you think?<|||||>Thanks for the suggestion. It makes sense. If backward-compatibility wasn't a big issue, I think it would be better to use `current_spm` (set to None) in all tokenizer methods and switch it to source or target spm using two (`as_source_tokenizer` and `as_target_tokenizer`) context managers as needed. This way encoding and decoding methods become source/ target agnositc. 
However, since it's an issue, I think your first suggestion which is using `use_source_tokenizer` is still better than setting an internal value from the user perspective because now they have to use a context_manager during encoding but then set an attribute during decoding which can persist during next decoding if not unset. What do you think?<|||||>> than setting an internal value from the user perspective

I never meant the user would have to set it. I meant for us to set it in `decode`/`decode_batch` with the `use_source_tokenizer` argument received.<|||||>Gotcha. Yeah, that should work.<|||||>Hi, the PR is ready for review. please let me know if the changes look good. Thanks.<|||||>Ok I think these changes address the comments. If there are still improvements to the docstring/ code you want to make, please feel free to push directly to this branch. Thanks.<|||||>The docstrings don't accumulate when you subclass and overload, they get reset. So we have to copy the whole things *and* add the extra argument. Will push on your branch the change.<|||||>Thanks. I think the PR is ready for the final review.
cc: @sgugger @patrickvonplaten @patil-suraj
<|||||>Thanks a lot for your work on this PR!<|||||>Thanks a lot for your feedback and a great PR experience!