Non-English characters not fully supported by GPT-2 HF model
## Environment info
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- `transformers` version:
- Platform: Ubuntu 20.04
- Python version: 3.7.11
- PyTorch version (GPU?):
- Tensorflow version (GPU?): yes
- Using GPU in script?:
- Using distributed or parallel set-up in script?:

### Who can help
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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @Rocketknight1

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

Model hub:

- for issues with a model report at https://discuss.huggingface.co/ and tag the model's creator.

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

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## Information

Model I am using (Bert, XLNet ...): GPT-2

The problem arises when using:
* [ ] the official example scripts: (give details below)
* [x] my own modified scripts: (give details below)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [x] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. @patrickvonplaten, @LysandreJik When I try to use the bad_words_ids argument in the generate method, it only takes in a list of ints (per the documentation: transformers.NoBadWordsLogitsProcessor). But if I've trained the GPT-2 model using any characters higher than UTF-8, the encoding for those characters requires more than just an int to represent. Is there any way to use Huggingface transformers for a language other than English and prevent the output from containing those characters as well? Or is there any alternative way to use GPT-2 with UTF-32 that you know of?

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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->

GPT-2 would be more usable if it could be fully supporting of UTF-32 characters, and not just UTF-8.
The `bad_word_ids` argument takes as input the token IDs, it's unrelated to the encoding. If you've trained a GPT-2 model on a larger vocabulary containing characters from other languages, then you should be able to specify the ID of the words you'd like to ignore in your vocabulary.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.