Add bash script to run all data preprocessing

[nik-sm]

Thanks for making this project open source.

Since it took some time to manually follow the data preprocessing instructions that you provided, I found it useful to write a single bash script to run all the preprocessing, and thought this might be convenient for others as well.

I also found that some minor changes to requirements.txt were necessary.

If this script would be more useful with changes, please let me know and I'll be happy to modify.

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[nik-sm]

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[lamblin]

Thanks for the contribution!
I would prefer to keep the minimum of files in the root of the repo, do you think there would be a way of making the script work from inside meta_dataset/dataset_conversion/ for instance?
Also, could you add a pointer to that script and how to use it in doc/dataset_conversion.md, and maybe mention it in README.md as well?