refactor(batch): change the number of tasks generated by batch read to half of batch_parallelism

[wcy-fdu]

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[liurenjie1024]

LGTM, thanks!

[wcy-fdu]

This change will make end to end source test timeout, specifically Extract data from kafka will spend take 3 minutes before the change, and 14 minutes after the change.

Do you think this is acceptable? Or we only apply this change for the new file source batch reading?
cc @tabVersion @chenzl25

[tabVersion]

does it make sense we load so much data in CI? cc @lmatz

[chenzl25]

I think we can just keep it unchanged. Or only apply this change to the file source.

[wcy-fdu]

I think we can just keep it unchanged. Or only apply this change to the file source.

After some offline discussion, we(@st1page @tabVersion) feel that there is no need to differentiate between Kafka or file source batch read here. Instead, we can use a magic number (currently set to 32) as the minimum value for the number of tasks, that is to set task parallelism = max(batch_parallelism / 2), 32.

[wcy-fdu]

merged into #15358