Decreasing peak memory usage for beam.TupleCombineFn

[iindyk]

Loading and merging accumulators in batches in beam.TupleCombineFn.merge_accumulators instead of loading and merging all at once.

This is a modification of 292dc44 with a fix of failing internal tests. The original change was reverted in 8c6ade2.

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ValidatesRunner compliance status (on master branch)

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

Examples testing status on various runners

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Java	---		---	---	---	---	---
Python	---	---	---	---	---	---	---
XLang	---	---	---	---	---	---	---

Post-Commit SDK/Transform Integration Tests Status (on master branch)

Go	Java	Python

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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[codecov]

Codecov Report

Merging #15494 (fde2fd7) into master (549d138) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #15494      +/-   ##
==========================================
+ Coverage   83.75%   83.77%   +0.02%     
==========================================
  Files         443      443              
  Lines       60085    60128      +43     
==========================================
+ Hits        50323    50374      +51     
+ Misses       9762     9754       -8     

Impacted Files	Coverage Δ
sdks/python/apache_beam/transforms/combiners.py	93.88% <100.00%> (+0.29%)	⬆️
...hon/apache_beam/runners/direct/test_stream_impl.py	94.02% <0.00%> (-2.24%)	⬇️
sdks/python/apache_beam/pipeline.py	91.66% <0.00%> (ø)
sdks/python/apache_beam/transforms/util.py	95.81% <0.00%> (ø)
...hon/apache_beam/portability/api/beam_fn_api_pb2.py	100.00% <0.00%> (ø)
sdks/python/apache_beam/coders/typecoders.py	94.52% <0.00%> (+0.07%)	⬆️
sdks/python/apache_beam/coders/coders.py	88.58% <0.00%> (+0.59%)	⬆️
sdks/python/apache_beam/io/source_test_utils.py	89.86% <0.00%> (+1.38%)	⬆️
.../python/apache_beam/transforms/periodicsequence.py	98.36% <0.00%> (+1.63%)	⬆️
... and 1 more

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[iindyk]

R: @robertwb

[robertwb]

It doesn't look like there's an easy way to compare the old and new versions, but 292dc44...iindyk:fde2fd763a4bf7e01810821c8c13e754cec32dad does the job

[robertwb]

Run Python PreCommit