feat: make python map_batches safer

[ritchie46]

This will default to a safer version of map_batches. We will now assume you cannot run on batches. E.g. don't merely do an elementwise operation like + or pow, but do something that requires all data. E.g. sort, sum, rolling_min, etc.

map_batches will now not run by default on the streaming engine unless you set is_elementwise=True. In that case it can produce wrong results in a group-by, but hey, you said it was elementwise. 🤷

[ritchie46]

@MarcoGorelli deanm0000 FYI

[avimallu]

@ritchie46, the explanation you gave in the issue isn't quite clear to me. I also think the documentation explanation should better explain what is_elementwise does, and explain somehow what a batch is in map_batches for this argument.

For example, the obvious next question that the user would think is "Why does map_batches not work in streaming mode by default? What the tradeoffs in me setting this argument to True?"

[ritchie46]

Because some functions are not correct on a subset of the data. E.g. a sort needs all data to be correct.