Get rid of unnecessary cogbk when running offline detector.

[shunping]

Improve the performance of offline detectors by putting the original example in the key and removing unnecessary CoGroupByKey step.

[github-actions]

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[github-actions]

R: @jrmccluskey for final approval

[shunping]

The failure on ProperyTestingCoders.test_row_coder seems unrelated.

[damccorm]

Thanks!