Fix `skip_special_tokens` for `Wav2Vec2CTCTokenizer._decode`
Author: msublee
Pull request: yes

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Fixes #29243 


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@ArthurZucker 

Comments:
I'm unfamiliar with this kind of PR (to big repo). Where and How should I add a small test?

I updated the test!
Also, I excluded the `pad_token` filtering in `_decode` since `pad_token` is used as CTC-blank token and is filtered in `convert_tokens_to_string`.
https://github.com/huggingface/transformers/blob/b27aa206ddf3fe66b36db587603141b3d0379a82/src/transformers/models/wav2vec2/tokenization_wav2vec2.py#L321
Finally, I applied this update to both `Wav2Vec2Tokenizer` and `Wav2Vec2CTCTokenizer`.

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