[prism] Fix top for unfused execution. Move to register.

[lostluck]

Some fixes for transforms/top WRT portable runners.

• Enable top to run in non-fusing runners.
  • Top is very clever with it's accumulator encoding/decoding, doing it just in time.
  • This change fixes a bug where if no merges happen in the merge accumulators stage the type and encoder is unknown. This caused a failure if the runner doesn't fuse with the downstream extract output stage.
  • In particular, we now check if the data bundle exists when we have 0 elements, and if so, re-encode using the existing bundle.
• Move to the generic Register package instead of the legacy code generator variant.

See #27550 and #24789.

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[codecov]

Codecov Report

Merging #27585 (bd183f8) into master (b2e00ef) will decrease coverage by 0.02%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master   #27585      +/-   ##
==========================================
- Coverage   71.14%   71.12%   -0.02%     
==========================================
  Files         861      859       -2     
  Lines      104560   104495      -65     
==========================================
- Hits        74389    74324      -65     
+ Misses      28621    28613       -8     
- Partials     1550     1558       +8     

Flag	Coverage Δ
go	53.57% <16.66%> (-0.09%)	⬇️

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Impacted Files	Coverage Δ
sdks/go/pkg/beam/transforms/top/top.go	74.10% <16.66%> (-1.99%)	⬇️

... and 16 files with indirect coverage changes

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[riteshghorse]

LGTM, thanks!

[riteshghorse]

More of an understanding question if a.data could also be assigned to nil as we do for a.list after this if block

[lostluck]

That's a very good question.

My instinct is that since this function is supposed to write and encode data, it shouldn't be mutating the value at all.

Which means technically it shouldn't be nil'ing out that list field at all either.

In principle, the encoding will only happen when emitting the values downstream, which should only happen once per given value, at which point the value itself should be garbage collected away anyway.

So I'm going to do the opposite: remove the nil on encode line there.