[Python] Expand SDF by default in PortableRunner

[Eliaaazzz]

Fixes #24422.

Python iobase.Read uses a Splittable DoFn internally. PortableRunner's default batch pre-optimization path did not include translations.expand_sdf, so portable runners that do not support SDFs natively can receive a single SDF ParDo for reads. In Spark, that means Python reads such as ReadFromParquet may execute on a single partition without parallelization.

This change:

• adds translations.expand_sdf to PortableRunner's default optimization phases
• allows --experiments=pre_optimize=expand_sdf as an explicit custom phase
• adds optimizer coverage for:
  • default SDF expansion
  • explicit pre_optimize=expand_sdf
  • beam.io.Read(BoundedSource) expansion, including the expected RESHUFFLE

The bounded read test covers the issue scenario behind ReadFromParquet and similar Python Read transforms by verifying that the optimized pipeline contains the SDF component stages:

• PAIR_WITH_RESTRICTION
• SPLIT_AND_SIZE_RESTRICTIONS
• PROCESS_SIZED_ELEMENTS_AND_RESTRICTIONS
• RESHUFFLE

Local verification:

• git diff --check
• python -m compileall sdks/python/apache_beam/runners/portability/portable_runner.py sdks/python/apache_beam/runners/portability/portable_runner_test.py

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical issue in Apache Beam's PortableRunner where Python iobase.Read operations, which internally utilize Splittable DoFns (SDFs), were not being properly parallelized on certain portable runners like Spark. The fix involves integrating the translations.expand_sdf optimization phase into the PortableRunner's default pipeline optimization process. This ensures that SDFs are correctly expanded into their constituent stages, thereby enabling parallel execution for read transforms and preventing performance bottlenecks, particularly for operations like ReadFromParquet.

Highlights

• Default SDF Expansion: The expand_sdf optimization phase was added to the PortableRunner's default pre-optimization path to ensure proper parallelization of iobase.Read operations.
• Explicit SDF Expansion: Users can now explicitly enable SDF expansion using the --experiments=pre_optimize=expand_sdf flag.
• Enhanced Test Coverage: New tests were introduced to verify the correct expansion of SDFs in both default and explicit optimization scenarios, including beam.io.Read(BoundedSource) with expected RESHUFFLE for parallelization.

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[github-actions]

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R: @damccorm for label python.

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