Fix backward compatibility in Dataset.load_from_disk
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` ?
This way future datasets can also be reloaded from older versions of the lib

`_indices_files` was introduced in a recent PR and was not released Yes, I have seen it is not released yet...

You 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`. ;)
Fix backward compatibility when loading from disk an old dataset saved to disk with indices using key "_indices_data_files".

Related to #2195.