Propagate annotations of CombinePerKey transforms to resulting component transforms

[hjtran]

Issue:

If somehow you have an annotation(s) on a CombinePerKey transform (e.g. because you subclass it and add annotations or a runner adds annotations), the annotations end up disappearing after getting translated by lifted_stages.

Considerations:

I don't think it's super obvious what the correct behavior is here. It's a bit surprising the annotation disappears but it may also be surprising for someone who meant to signal some behavior for one transform to suddenly have it for the four resulting component transforms. I put up this PR in case it does make sense to propagate the annotations since for our use case it simplifies things a lot

Context:

We're writing a runner and we'd like to be able to save the top level user labels that users assign to their transforms so we can nicely log which user transform is being run at every phase. The most obvious way to me to do this was to create some custom annotations and then search for them at phase execution time. I was surprised when the CPK annotations were missing. Open to other ways to do this though

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[hjtran]

R: @robertwb

[github-actions]

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

[robertwb]

Thanks. LGTM once yapf/lint are happy.

[codecov]

Codecov Report

Merging #28489 (b3a8e15) into master (38720a3) will increase coverage by 0.00%.
Report is 4 commits behind head on master.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #28489   +/-   ##
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  Coverage   72.22%   72.23%           
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  Files         684      684           
  Lines      100856   100855    -1     
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+ Hits        72846    72848    +2     
+ Misses      26434    26431    -3     
  Partials     1576     1576           

Flag	Coverage Δ
python	82.82% <ø> (+<0.01%)	⬆️

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Files Changed	Coverage Δ
.../runners/portability/fn_api_runner/translations.py	96.79% <ø> (ø)

... and 6 files with indirect coverage changes

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[hjtran]

Thanks. LGTM once yapf/lint are happy.

Done