[GPT2Tokenizer] Raise ValueError for Fast GPT2Tokenizer with bos token for now
Author: patrickvonplaten
Pull request: yes

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@sgugger - As discussed offline the best fix here will be to make sure GPT2TokenizerFast works correctly which is however dependent on https://github.com/huggingface/tokenizers/pull/1005 and will probs take some time. Think it's important that we raise a ValueError though as otherwise users will run into silently not adding BOS to OPT which I'd like to avoid.

The error message should be clear enough for the user to understand how to change. 

@LysandreJik @sgugger also make it's worth doing a patch release here actually (not sure maybe not super important though)


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Comments:
cc @thomasw21 @SaulLu @mishig25 @ArthurZucker 

_The documentation is not available anymore as the PR was closed or merged._

Thanks @patrickvonplaten ! It seems really good to have this error raised while figuring out how to do the processor.

> Also make it's worth doing a patch release here actually (not sure maybe not super important though)

We're likely going to release tomorrow or as soon as BLOOM is merged, so this will be included in it!

Hello! when using the GPT2TokenizerFast for the OPT model, I get a warning that the fast version of the tokenizer is working incorrectly. I'm wondering what is the status of this problem, or is it okay to use the fast tokenizer now?


I think the fast OPTTokenizer should work now after it has been merged to `tokenizers` no? cc @Narsil 

Actually no need for any modifications or `tokenizers` specific version. Everything should have been fixed within `transformers`. 

But I am not sure about every single OPT model on the hub, I didn't make modifications there (nor am I sure how I should do that).
So if we are using an old incorrect tokenizer, the warning might still be valid.