Fn Runner Watermark issue

[13MaxG]

Proposing a solution to https://github.com/apache/beam/issues/26190 . It appears the Flatten was not setting a watermark, which caused following steps not to execute.

The issue was returning errors on beam versions 2.39 onwards, and it potentially produced unstable results before 2.39.

There are meaningful TODOs mentioned: # TODO(robertwb): Possibly fuse multi-input flattens into one of the stages. # TODO(pabloem): Consider case where there are various producers

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Reminder, please take a look at this pr: @claudevdm @damccorm

[damccorm]

waiting on author

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