Support complex feature types as `features` in packaged loaders 
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2022-05-17T18:06:49Z
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2022-05-31T12:26:23Z
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This PR adds `table_cast` to the packaged loaders to fix casting to the `Image`/`Audio`, `ArrayND` and `ClassLabel` types. If these types are not present in the `builder.config.features` dictionary, the built-in `pa.Table.cast` is used for better performance. Additionally, this PR adds `cast_storage` to `ClassLabel` to support the string to int conversion in `table_cast` and ensure that integer labels are in a valid range.

Fix https://github.com/huggingface/datasets/issues/4210

This PR is also a solution for these (popular) discussions: https://discuss.huggingface.co/t/converting-string-label-to-int/2816 and https://discuss.huggingface.co/t/class-labels-for-custom-datasets/15130/2

TODO:
* [x] tests