Add an annotation to expose transforms to yaml.

[robertwb]

We should add this to all transforms that are simply parameterized.

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Mention the appropriate issue in your description (for example: addresses #123), if applicable. This will automatically add a link to the pull request in the issue. If you would like the issue to automatically close on merging the pull request, comment fixes #<ISSUE NUMBER> instead.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

To check the build health, please visit https://github.com/apache/beam/blob/master/.test-infra/BUILD_STATUS.md

GitHub Actions Tests Status (on master branch)

See CI.md for more information about GitHub Actions CI or the workflows README to see a list of phrases to trigger workflows.

[robertwb]

R: @Polber

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[codecov]

Codecov Report

Merging #28208 (c7177b9) into master (45ea758) will increase coverage by 0.06%.
Report is 126 commits behind head on master.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master   #28208      +/-   ##
==========================================
+ Coverage   72.30%   72.36%   +0.06%     
==========================================
  Files         678      679       +1     
  Lines       99853   100308     +455     
==========================================
+ Hits        72197    72591     +394     
- Misses      26077    26138      +61     
  Partials     1579     1579              

Flag	Coverage Δ
python	82.90% <66.66%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
sdks/python/apache_beam/transforms/ptransform.py	92.73% <66.66%> (-1.06%)	⬇️

... and 21 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[Polber]

Will this replace the annotation assembly performed in yaml_transform.py? This would require all transforms used in a YAML pipeline have this annotation, right?

[Polber]

Is the print meant to still be here?

[robertwb]

good catch

[robertwb]

This won't replace the assembly performed in `yaml_transform.py, this merely gives indications of how to use other (posibly not-directly-exposed) Beam transforms directly from yaml.

[robertwb]

Any concerns about this?

[Polber]

No concern, just wondering the use-case since the annotations get added in yaml_transform, but this makes sense for custom transforms/ those not exposed to yaml as you said. LGTM