Added preprocessing.mdx italian translation
Author: nickprock
Pull request: yes

## What does this PR do?

Italian translation of doc related to the preprocessing of :hugs: Transformers.
* updated _toctree.yml
* added preprocessing.mdx

## Before submitting
- [X] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [X] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [X] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [X] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).

See issue: [#17459](https://github.com/huggingface/transformers/issues/17459)

@omarespejel 
@sgugger 

Comments:
_The documentation is not available anymore as the PR was closed or merged._

Hi @nickprock! Thank you for this translation 🚀🚀
Can I ask you if you can edit these things?
- Un modello non comprende il testo piano --> un modello non comprende un testo grezzo
-  CLS and SEP (classifier and separator) - --> CLS e SEP (classificatore e separatore)
- just before the "Audio" chapter there's a problem if you see the doc preview, where it says: ```py >>> batch_sentences = [ ... "But what about second breakfast?", ... "Don't think he knows about second breakfast, Pip.", ... "What about elevensies?", ... ] >>> encoded_input = tokenizer(batch_sentences, padding=True, truncation=True, return_tensors="tf") >>> print(encoded_input) {'input_ids': , 'token_type_ids': , 'attention_mask': } ``` I see the same problem in the English doc, you can take a cue from the Spanish doc where the problem has been solved
- E' importante che la frequenza --> È importante che la frequenza
- il modello Wav2Vec2 msu questo dataset --> il modello Wav2Vec2 su questo dataset
- Usa il metodo in 🤗 Datasets' cast_column per alzare della frequenza di campionamento a 16kHz --> Usa il metodo di 🤗 Datasets cast_column per alzare la frequenza di campionamento a 16kHz
- uno 0 - interpretato come silenzaio --> uno 0 - interpretato come silenzio
- Pad and truncate --> Pad e truncate
- il primo campione ga una sequenza --> il primo campione ha una sequenza
- LCrea una funzione che preprocesserà il dataset --> Crea una funzione che preprocesserà il dataset
- per applicare al volo la trasformzazione --> per applicare al volo la trasformazione
- Il Tokenizer per processarfe i testi. --> Il Tokenizer per processare i testi.
- e tokenizza il in to labels --> e tokenizza il testo in labels
- Fantastico, ora dovreste -> Fantastico, ora dovresti

Thank you 🌈🌈

@mfumanelli thanks for the review 😊 

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/main/CONTRIBUTING.md) are likely to be ignored.

I fixed and I'm waiting the review

cc @omarespejel