Fix backward compatibility in Dataset.load_from_disk
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['Hi @lhoestq, could you please check if this makes sense? Thanks.'
 'What about using `_indices_data_files` field in save_to_disk instead of `_indices_files` ?\r\nThis way future datasets can also be reloaded from older versions of the lib\r\n\r\n`_indices_files` was introduced in a recent PR and was not released'
 'Yes, I have seen it is not released yet...\r\n\r\nYou are right! It was your awesome PR on Tables which renamed this. If there is no particular reason for this renaming, yes, we could switch it back to the previous `_indices_data_files`. ;)']
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Fix backward compatibility when loading from disk an old dataset saved to disk with indices using key "_indices_data_files".

Related to #2195.
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