[BEAM-11472] Support SDF on non-splitting runners such as legacy Dataflow.

[robertwb]

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Post-Commit Tests Status (on master branch)

Lang	SDK	Dataflow	Flink	Samza	Spark	Twister2
Go		---		---		---
Java
Python				---		---
XLang				---		---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website	Whitespace	Typescript
Non-portable
Portable	---		---	---	---	---

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GitHub Actions Tests Status (on master branch)

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[robertwb]

R: @TheNeuralBit

Successful job at https://pantheon.corp.google.com/dataflow/jobs/us-central1/2020-12-15_14_51_13-12368891253353011788;step=?project=apache-beam-testing

[TheNeuralBit]

LGTM, thank you!

Do you know if this worked at some point and was broken?

[robertwb]

I never was able to find the point at which this was broken (though I didn't look that far back). It could have been quite far back (e.g. replaced with the dynamic splitting capabilities, rather than augmented by it).