Blank token id hard coded in C++ decoders

[zhr1201]

Describe the bug
Following the PR that adds whisper feature extraction #2320. The transcript produced in that PR contains lots of <|notimestampes|>.
This is because i exported whisper tokenizers with 99 languages, but for whisper large v3, tokenizer it's 100 langs. And the token corresponding to the extra output in the transcript is actually <|nospeech|>, which is used as the blank token in CTC. Fixing the token list does produce reasonable output, but still significantly worse than the one produced by transcribe.py that simulates a streaming inference.

I think there is still some mismatch in the decoder part, at least It's not possible to pass in blank token id through CLI.
For ctc prefix beam search, there is a field in the config

wenet/runtime/core/decoder/ctc_prefix_beam_search.h

Line 30 in 4c81459

int blank = 0; // blank id

, but not passed in through CLI

wenet/runtime/core/decoder/params.h

Line 75 in 4c81459

// DecodeOptions flags

and for wfst decoder, it assume the first token is blank.

wenet/runtime/core/decoder/ctc_wfst_beam_search.cc

Line 80 in 4c81459

float blank_score = std::exp(logp[i][0]);

To Reproduce
As described above

Expected behavior
CLI can handle blank tokens for different models, not just the case where the first token is blank.

[Bonus] decoder main produces the same result as transcribe.py for streaming finetuned whisper trained with hybrid ctc+attention loss

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[robin1001]

Yes, it's by design and it works fine before. It is a problem for whisper pretrained model since there is no blank in whisper tokenizer.

[robin1001]

@xingchensong any comment?

[xingchensong]

we should make it configurable like this :

• feat(decode): support arbitrary blank_id during decoding #2193
• fix(whisper): support arbitrary ctc blank id #2157