DFDataset: do not pickle sharding info

[NeoLegends]

... but rely on _distrib_info instead.

Fixes #1678

@Judyxujj feel free to give this PR a test run.

[albertz]

I don't understand. We actually want that this is pickled/serialized. It doesn't make sense to not serialize this? We want/need this information.

[albertz]

How does this work together with #1676?

[NeoLegends]

I don't understand. We actually want that this is pickled/serialized. It doesn't make sense to not serialize this? We want/need this information.

The dataset already handles the pickling of this information on its own via _distrib_info and then makes sure the user does not set the properties by hand. Alternatively we can remove that property and the assertion. I went the other way now because it seems like a good-enough fix that keeps the existing assertion intact until #1676 lands.

How does this work together with #1676?

#1676 removes the assertion, so the change here won't be necessary anymore.

[albertz]

So instead of removing/cleaning up the obsolete/duplicate code/logic with _distrib_info, you add another workaround? I think cleaning up makes much more sense than adding yet another workaround?

[NeoLegends]

If it unblocks @Judyxujj quickly then I see value in that, it's not like this PR has a huge number of lines :D

[NeoLegends]

Superseeded by #1676