wav2vec2: support datasets other than LibriSpeech
# What does this PR do?
Building on #10145, I'm adding support for the two other speech datasets (besides LibriSpeech) for ASR at the time of writing (`timit_asr` and `arabic_speech_corpus`), which require the following:
* Custom validation split name
* On-the-fly resampling support to target feature extractor's sampling rate (via `librosa` -- see `requirements.txt`)
* Max duration (in seconds) filter to remove outliers (which may crash GPU training when running OOM)
* Verbose logging to help debug custom datasets, including reverse transliteration (via `lang-trans` -- see `requirements.txt`)
* Pre-processing: tokenization and text normalization using orthography rules
  * Casing (`do_lower_case`)
  * Custom vocab for tokens used in orthography (e.g., Buckwalter transliteration for Arabic)
  * Custom word delimiter token (when the default, `"|"`, is used in orthography)
  * Transformations similar to those in `jiwer` to normalize transcripts for training
  * Removing special words (e.g., "sil" which can be used to indicate silence)
  * Translation table (e.g., "-" -> " " to break compounds like "quarter-century-old")
  * Cleaning up characters not in vocab (after applying the rules above)

Arabic model: https://huggingface.co/elgeish/wav2vec2-large-xlsr-53-arabic
TIMIT models: https://huggingface.co/elgeish/wav2vec2-base-timit-asr and https://huggingface.co/elgeish/wav2vec2-large-lv60-timit-asr

## Who can review?
@patrickvonplaten @sgugger @LysandreJik @patil-suraj @SeanNaren

Thanks to a fix for `timit_asr` that @patrickvonplaten made, now I have some good results using `wav2vec2-base`:

<img width="1050" alt="timit_asr_pr_ 10581" src="https://user-images.githubusercontent.com/6879673/110573935-0435e480-8111-11eb-8e0e-845af4e2eab7.png">

I'm running one for `arabic_speech_corpus` using `wav2vec2-base` as well. @patrickvonplaten let me know when you have the rest of configs for https://huggingface.co/facebook/wav2vec2-large-xlsr uploaded so I can try it as well (or a workaround). Thanks!<|||||>Thanks for adding this functionality! Your script worked succesfully when I fine-tuned `wav2vec2-base` , `xlsr` (https://dl.fbaipublicfiles.com/fairseq/wav2vec/xlsr_53_56k.pt) and `wa2vec2-large-100k` (multilingual Large Model from https://github.com/facebookresearch/voxpopuli#pre-trained-models pre-trained on VoxPopuli dataset) on TIMIT dataset. If fine-tuning on some another custom dataset, is it enough to set `--orthography`  to `timit`  in `run_asr.py` if the transcriptions are lowercased and `librispeech` if they are uppercased?<|||||>> Thanks for adding this functionality! Your script worked succesfully when I fine-tuned `wav2vec2-base` , `xlsr` (https://dl.fbaipublicfiles.com/fairseq/wav2vec/xlsr_53_56k.pt) and `wa2vec2-large-100k` (multilingual Large Model from https://github.com/facebookresearch/voxpopuli#pre-trained-models pre-trained on VoxPopuli dataset) on TIMIT dataset.

Thanks, @Getmany1 - you can do me a favor and run it with `arabic_speech_corpus` dataset and `--target_feature_extractor_sampling_rate --orthography buckwalter` on `xlsr` to verify it works with extended vocab. Unfortunately I can't fit `xlsr` on my machine.

> If fine-tuning on some another custom dataset, is it enough to set `--orthography` to `timit` in `run_asr.py` if the transcriptions are lowercased and `librispeech` if they are uppercased?
For the most part, yes! Run it with `--verbose_logging` to see how the orthography rules pre-processed the text. Keep us posted!<|||||>`XLSR `model I used didn't work with this setup: the training loss is _nan_ when I tried to fine-tune on Arabic corpus. If I got it correctly, with `--orthography buckwalter` you modify the tokenizer only. However, if you load e.g. 
`model = Wav2Vec2ForCTC.from_pretrained("facebook/wav2vec2-large-xlsr-53")`
and check it structure
`print(model.state_dict)`
you'll see that the last layer of the network is the LM head with the default vocabulary size:
`(lm_head): Linear(in_features=1024, out_features=32, bias=True)`
If I understand correctly, you need to convert the model manually if you want to have a letter vocabulary different from english.
I converted the fairseq xlsr checkpoint using this script
`transformers/src/transformers/models/wav2vec2/convert_wav2vec2_original_pytorch_checkpoint_to_pytorch.py`
together with "custom", Swedish letter dictionary, and then succeeded to fine-tune it on a Swedish corpus.
I guess you need to do the same for Arabic in order to have proper LM head on top of the model.<|||||>Yes, thanks! I missed that step (which is similar to `PreTrainedModel.resize_token_embeddings()`). I'm adding a method for that: `Wav2Vec2ForCTC.resize_lm_head()` which simply calls `PreTrainedModel.model._get_resized_lm_head()` and updates model config. The LM head looks good when inspecting the return value. I'll add some unit tests as well.

Now I'm fine-tuning it on `wav2vec2-base` but I'm not expecting great results given the phonetic differences with Arabic. If you can try it again with `xlsr`, I'd appreciate it!<|||||>Thanks @elgeish, the method seems to work correctly. Now the loss is not `nan` any more during the fine-tuning of the `xlsr` model. Training loss:

```
{'loss': 584.0067, 'learning_rate': 8.333333333333333e-05, 'epoch': 0.28}
{'loss': 291.7098, 'learning_rate': 0.00016666666666666666, 'epoch': 0.55}
```
Validation loss:
```
{'eval_loss': 374.4364929199219, 'eval_wer': 1.0, 'eval_runtime': 22.3424, 'eval_samples_per_second': 4.476, 'epoch': 0.28}
{'eval_loss': 374.20855712890625, 'eval_wer': 1.0, 'eval_runtime': 22.8504, 'eval_samples_per_second': 4.376, 'epoch': 0.55}
```
Predictions after epoch 0.55:
```
03/12/2021 12:07:54 - DEBUG - __main__ -   reference: "wayaquwlu lEulamA'u <in~ahu min gayri lmuraj~aHi >an tuTaw~ira lbaktiyryA lmuEdiyapu muqAwamapan Did~a lEilAji ljadiyd >al~a*iy >aSbaHa mutAHan biAlfiEl fiy $akli marhamin lil>amrADi ljildiy~api"
03/12/2021 12:07:54 - DEBUG - __main__ -   prediction: ""
03/12/2021 12:07:54 - DEBUG - __main__ -   reference: "wayumkinuka lHuSuwlu EalY taTbiyqAtin lilt~adriybAti l>asAsiy~api maj~Anan"
03/12/2021 12:07:54 - DEBUG - __main__ -   prediction: ""
```

Unfortunately, after that I always run out of memory in Colab. The recordings in the Arabic corpus are probably very long. As a result even with `batch_size=1` 16Gb of GPU memory is not enough:
```
RuntimeError: CUDA out of memory. Tried to allocate 80.00 MiB (GPU 0; 15.90 GiB total capacity; 14.80 GiB already allocated; 87.75 MiB free; 14.94 GiB reserved in total by PyTorch)
  2% 1050/54390 [07:16<6:09:57,  2.40it/s]
```
If you have more than 16Gb of GPU memory available, I can share the `xlsr` model to try it out on your machine.<|||||>I added a `--max_duration_in_seconds` filter. I'm seeing ok results now fine-tuning `wav2vec2-base` after 500 steps, for example:
```
reference: "wamin tilka ls~ilaE >al$~Ayu lS~iyniy~u wAlwaraqu wAlbAruwdu wAlbuwSilapu"
prediction: "wamin tiloka Als~ila>a$~aAyu AS~iyniy~u walowaraqu waAlobaAruwdu waAlobuwSilapu"
```
I've also started fine-tuning [wav2vec2-large-xlsr-53](https://huggingface.co/facebook/wav2vec2-large-xlsr-53) after adding missing files locally on my machine.

For future PRs, I'm thinking of:
* Supporting other languages and examples from [Patrick's awesome blog post](https://huggingface.co/blog/fine-tune-xlsr-wav2vec2), which I need to read
* CER and WER transformations for other languages (e.g., ignoring tashkil in Arabic)
* Supporting [Lhotse](https://lhotse.readthedocs.io/en/latest/corpus.html) datasets, which provide a ton of speech-specific functionality

Before adding UTs, I want to check with @patrickvonplaten the code here is on the right track.
@Getmany1 you've been super helpful, thank you!<|||||>> Hey @elgeish,
> 
> Thanks a lot for your PR! Sorry that I only reviewed it now. In general I like the changes you have made to `run_wav2vec2.py` :-)
> 
> A couple of things, I'd like to change:
> 
> * Remove the change from `modeling_wav2vec2.py`. I don't really want a resize lm head method in the model there -> it's too much of an edge case IMO. You can simply first instantiate the processor then use the tokenizer length to load the model with `Wav2Vec2ForCTC.from_pretrained(model_path, vocab_size=len(tokenizer))`. This way we don't need a `resize_lm_head()` method.
> * Can you give me more context on the `buckwalter.json` file?
> * Can you also add more text to the README.md that explains a bit how to use the `run_wav2vec2.py` script? E.g. what does the orthography class do, what is the buckwalter vocab?

Thank you! I responded inline. I'll update `README.md` as well. I think you mean `run_asr.py`, no?<|||||>Great the PR looks good to me now! Thanks a lot for doing this :-) The failures seem unrelated, so I'll rerun the tests again to make the CI happy