Flax Wav2Vec2 - Add venv section and fix training script
Author: mariagrandury
Pull request: yes

# What does this PR do?

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Training Facebook's Wav2Vec2 as explained in this README, I encountered several issues related to:
- libraries that should be installed to support audio data and
- the proposed training script.

The purpose of this PR is to explain which libraries could be helpful to work with audio data and to update the current training script so it can be used out-of-the-box to train a Wav2Vec2 model.

The error messages that motivated each of the changes in this PR are listed [here](https://github.com/nlp-en-es/wav2vec2-spanish/blob/main/differences_from_original.md).

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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      to it if that's the case.
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      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

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members/contributors who may be interested in your PR.

I think @patrickvonplaten would be the right person to review this PR.

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