combine should be parallelizable in many cases

[damccorm]

Relevant discussion: https://lists.apache.org/thread.html/r9e7d9527eb1d4c9c097c91c010a25dabf4a5f8053d50dc3b6d90d36a%40%3Cdev.beam.apache.org%3E

Currently we require Singleton partitioning for combine() because func might operate on the full dataset, but in many cases func is actually an elementwise method. We should detect this when possible (e.g. when func is an np.ufunc), and/or provide a flag to let the user indicate the function is elementwise.

Imported from Jira BEAM-12351. Original Jira may contain additional context.
Reported by: bhulette.