Make PeriodicImpulse generates unbounded PCollection

[Abacn]

Fixes #21606.

• DoFn generates unbounded pcoll if marked by unbounded_per_element

• Mark ImpulseSeqGenDoFn.process as unbounded_per_element

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[Abacn]

R: @lostluck

[lostluck]

This LGTM, but I am not a python expert. That's @robertwb. I'll merge in tomorrow unless Robert chime's in first.