Fix flatten of complex feature types
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Apparently the merge brought back some tests that use `flatten_()` that we removed recently
2022-03-09T11:26:32Z
https://github.com/huggingface/datasets/pull/3723#issuecomment-1062822888
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_The documentation is not available anymore as the PR was closed or merged._
2022-03-17T15:51:55Z
https://github.com/huggingface/datasets/pull/3723#issuecomment-1071005370
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2022-03-18T17:32:26Z
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Fix `flatten` for the following feature types: Image/Audio, Translation, and  TranslationVariableLanguages.

Inspired by `cast`/`table_cast`, I've introduced a `table_flatten` function to handle the Image/Audio types.

CC: @SBrandeis  

Fix #3686.
