[query] warn on using randomness on lowered backends (#11672)
[query] warn on using randomness on lowered backends (#11672)

* [query] Support repartition on lowered backends

* support unkeyed

* warn on using randomness on service

* fixed

* fix no parts

* test_naive_coalesce passes now

* drop dummy key before dropping field

* Update service_backend.py

* Update py4j_backend.py

* Update backend.py

* Update ir.py

Co-authored-by: Tim Poterba <537603bd54b0293f70e81fdd35fbbf332ae5c194@gmail.com>
Co-authored-by: Daniel King <a3cd222ea982fef7e4d761fbe5435903b90070ac@broadinstitute.org>
Co-authored-by: Dan King <7739259b40d3ef66b5ccc726303499d46adb7c70@gmail.com>
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