Added Feature: Prefix decoding for wav2vec2 models
# What does this PR do?
Added the code for prefix decoding for wav2vec2 based models.
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Fixes #11283 


## Before submitting
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## Who can review?

@patrickvonplaten @patil-suraj 

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
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Library:

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- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

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

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- [x]  Currently the code supports prefix decoding without LM. I am still working to integrate the kenlm version.

Problem faced currently: I created a custom kenlm and tried to run the code, but it stops without throwing any error at line `results = self.decoder.decode(emissions_ptr, T, N)` I am currently trying to fix it. (RESOLVED)

- [x] Shall I create a .sh or .txt file to guide on how to install flashlight dependencies?<|||||>Performance: 
                      Model:- `facebook/wav2vec2-base-960h`
                      Dataset:- `timit_asr`, `clean`, `test[:5%]`
                      Viterbi Decoding:- `wer: 0.115`
                      KenLM Decoding:- `wer: 0.098` <|||||>Wuhuhu! This is an amazing contribution @deepang17 - Super exciting to merge this notebook :-) And yes, it would be great if you could add a section to the README.md that explains how to use your script + maybe with some results (using Prefix decoding vs. not using it on *e.g.* Timit_asr and/or Librispeech evaluation - kinda like you already did above). I'm also very happy to help you run some evals! <|||||>Thank you for the appreciation. I will do the required changes to `README.md` and push a commit soon.<|||||>> * [x]   Currently the code supports prefix decoding without LM. I am still working to integrate the kenlm version.
> 
> Problem faced currently: I created a custom kenlm and tried to run the code, but it stops without throwing any error at line `results = self.decoder.decode(emissions_ptr, T, N)` I am currently trying to fix it. (RESOLVED)
> 
> * [x]  Shall I create a .sh or .txt file to guide on how to install flashlight dependencies?

@deepang17
Did you pushed that fix? I've tried your code and it is crushing at the "self.decoder.decode". What was your fix?

What is the status of this PR?
<|||||>> > * [x]    Currently the code supports prefix decoding without LM. I am still working to integrate the kenlm version.
> > 
> > Problem faced currently: I created a custom kenlm and tried to run the code, but it stops without throwing any error at line `results = self.decoder.decode(emissions_ptr, T, N)` I am currently trying to fix it. (RESOLVED)
> > 
> > * [x]   Shall I create a .sh or .txt file to guide on how to install flashlight dependencies?
> 
> @deepang17
> Did you pushed that fix? I've tried your code and it is crushing at the "self.decoder.decode". What was your fix?
> 
> What is the status of this PR?

You can fix it by replacing `!cmake .. -DCMAKE_BUILD_TYPE=Release -DKENLM_MAX_ORDER=20 -DCMAKE_POSITION_INDEPENDENT_CODE=ON` to `!cmake ..`<|||||>> > > * [x]     Currently the code supports prefix decoding without LM. I am still working to integrate the kenlm version.
> > > 
> > > Problem faced currently: I created a custom kenlm and tried to run the code, but it stops without throwing any error at line `results = self.decoder.decode(emissions_ptr, T, N)` I am currently trying to fix it. (RESOLVED)
> > > 
> > > * [x]    Shall I create a .sh or .txt file to guide on how to install flashlight dependencies?
> > 
> > 
> > @deepang17
> > Did you pushed that fix? I've tried your code and it is crushing at the "self.decoder.decode". What was your fix?
> > What is the status of this PR?
> 
> You can fix it by replacing `!cmake .. -DCMAKE_BUILD_TYPE=Release -DKENLM_MAX_ORDER=20 -DCMAKE_POSITION_INDEPENDENT_CODE=ON` to `!cmake ..`

Can you please publish a Google Colab or a bash script to do the installation? I could't figure out where to do the change you suggested in the build, I'v used the Google Colab example from flashlight.<|||||>@deepang17 Thank you for your amazing work!
I made Google Colab to reproduce this pull request. @samuelazran You can check this.
https://colab.research.google.com/drive/1HHEBS3I4biQ8ZDyfJDtHi4E4onOtYe46?usp=sharing

Viterbi decoding works well, but KenLM decoding has the following error.

```
  File "run_wav2vec2_eval_with_lm.py", line 292, in <module>
    main()
  File "run_wav2vec2_eval_with_lm.py", line 281, in main
    results = selected_dataset.map(map_to_result)
  File "/usr/local/lib/python3.7/dist-packages/datasets/arrow_dataset.py", line 1606, in map
    desc=desc,
  File "/usr/local/lib/python3.7/dist-packages/datasets/arrow_dataset.py", line 176, in wrapper
    out: Union["Dataset", "DatasetDict"] = func(self, *args, **kwargs)
  File "/usr/local/lib/python3.7/dist-packages/datasets/fingerprint.py", line 397, in wrapper
    out = func(self, *args, **kwargs)
  File "/usr/local/lib/python3.7/dist-packages/datasets/arrow_dataset.py", line 1911, in _map_single
    example = apply_function_on_filtered_inputs(example, i, offset=offset)
  File "/usr/local/lib/python3.7/dist-packages/datasets/arrow_dataset.py", line 1826, in apply_function_on_filtered_inputs
    function(*fn_args, effective_indices, **fn_kwargs) if with_indices else function(*fn_args, **fn_kwargs)
  File "run_wav2vec2_eval_with_lm.py", line 265, in map_to_result
    decoder = W2lKenLMDecoder(eval_args, target_dictionary)
  File "run_wav2vec2_eval_with_lm.py", line 201, in __init__
    self.lm = KenLM(args.kenlm_model, self.word_dict)
TypeError: __init__(): incompatible constructor arguments. The following argument types are supported:
    1. flashlight.lib.text.flashlight_lib_text_decoder.KenLM(path: str, usr_token_dict: fl::lib::text::Dictionary)

Invoked with: None, <flashlight.lib.text.flashlight_lib_text_dictionary.Dictionary object at 0x7fe0ef7294b0>
```

@deepang17  Do you know this error?  It exactly gives as an argument of flashlight.lib.text.flashlight_lib_text_decoder.KenLM the dict obtained from flashlight.lib.text.dictionary.create_word_dict.<|||||>@deepang17 - do you have updates regarding the README.md script? :-) I can take over the PR by next week otherwise!<|||||>Hello @patrickvonplaten, Sorry for the delay. I was occupied due to some personal issues. I am on the verge of completing the README.md. I will commit the updated README soon.<|||||>@deepang17  Any updates?<|||||>@deepang17 Any updates? <|||||>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.<|||||>This PR seems to be stuck since quite some time now. Is anyone interested in finishing / testing this PR? 

Might be better to start fresh otherwise with a blog post ) colab that explains how to make a complete ASR end-to-end system - cc @anton-l <|||||>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.<|||||>@patrickvonplaten 
I'm in! I've searched this topic but it seems there is no official implementation on this topic, and It would be so nice to add this feature. If this feature is still in the backlog, I would be happy to contribute. Looking forward to your alert!<|||||>Hey @hbasafa,

I'm now working on this topic full time. 

We will most likely foster a closer collaboration between [pyctcdecode](https://github.com/kensho-technologies/pyctcdecode) and Transformers. [Here](https://github.com/patrickvonplaten/Wav2Vec2_PyCTCDecode) is a github repo that shows how to use `pyctcdecode` with Wav2Vec2 for LM supported decoding. It works quite well with KenLM.<|||||>Nice one! I will check it out.

As I was in hurry, I've already used [this code](https://github.com/hbasafa/py-ctc-decode) that could be easily installed via pip.
The code sample is also provided in [here](https://github.com/hbasafa/wav2vec_decode)
And Now I also focused on other decoding strategies to add there.

However, Thank you for sharing!
@patrickvonplaten<|||||>> Hey @hbasafa,
> 
> I'm now working on this topic full time.
> 
> We will most likely foster a closer collaboration between [pyctcdecode](https://github.com/kensho-technologies/pyctcdecode) and Transformers. [Here](https://github.com/patrickvonplaten/Wav2Vec2_PyCTCDecode) is a github repo that shows how to use `pyctcdecode` with Wav2Vec2 for LM supported decoding. It works quite well with KenLM.

hi @patrickvonplaten  - this is great news. Where is the best place to follow your progress? <|||||>This PR: https://github.com/huggingface/transformers/pull/14339

It all depends a bit on how fast we can merge a `load_from_hf_hub` function to `pyctcdecode`