Added type annotations to some combiners missing it.

[dmkozh]

These just correspond to the typing annotations of functions called in extend, so this shouldn't change the type check behavior.

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ValidatesRunner compliance status (on master branch)

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

Examples testing status on various runners

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Go	---	---	---	---	---	---	---
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
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Portable	---			---	---	---

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[dmkozh]

R: @robertwb

[codecov]

Codecov Report

Merging #15414 (d209b71) into master (cbb363f) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #15414      +/-   ##
==========================================
- Coverage   83.83%   83.75%   -0.09%     
==========================================
  Files         440      443       +3     
  Lines       59886    60099     +213     
==========================================
+ Hits        50207    50336     +129     
- Misses       9679     9763      +84     

Impacted Files	Coverage Δ
sdks/python/apache_beam/transforms/combiners.py	93.77% <100.00%> (+0.06%)	⬆️
sdks/python/apache_beam/utils/interactive_utils.py	92.68% <0.00%> (-2.44%)	⬇️
...hon/apache_beam/runners/direct/test_stream_impl.py	94.02% <0.00%> (-2.24%)	⬇️
...ks/python/apache_beam/runners/worker/data_plane.py	87.70% <0.00%> (-1.68%)	⬇️
.../python/apache_beam/transforms/periodicsequence.py	96.72% <0.00%> (-1.64%)	⬇️
sdks/python/apache_beam/io/source_test_utils.py	88.47% <0.00%> (-1.39%)	⬇️
sdks/python/apache_beam/runners/direct/executor.py	96.25% <0.00%> (-0.54%)	⬇️
...apache_beam/runners/dataflow/internal/apiclient.py	76.46% <0.00%> (-0.34%)	⬇️
...hon/apache_beam/runners/worker/bundle_processor.py	93.51% <0.00%> (-0.25%)	⬇️
setup.py	0.00% <0.00%> (ø)
... and 12 more

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[pabloem]

note there's one little formatting error: https://ci-beam.apache.org/job/beam_PreCommit_PythonFormatter_Commit/7939/console

[robertwb]

Can you fix the yapf errors?

19:46:42 py3-yapf-check run-test: commands[1] | time yapf --diff --parallel --recursive apache_beam
19:47:09 --- apache_beam/transforms/combiners.py	(original)
19:47:09 +++ apache_beam/transforms/combiners.py	(reformatted)
19:47:09 @@ -700,6 +700,7 @@
19:47:09          a in zip(self._combiners, accumulator)
19:47:09      ]
19:47:09  
19:47:09 +
19:47:09  @with_input_types(T)

[dmkozh]

Can you fix the yapf errors?

19:46:42 py3-yapf-check run-test: commands[1] | time yapf --diff --parallel --recursive apache_beam
19:47:09 --- apache_beam/transforms/combiners.py	(original)
19:47:09 +++ apache_beam/transforms/combiners.py	(reformatted)
19:47:09 @@ -700,6 +700,7 @@
19:47:09          a in zip(self._combiners, accumulator)
19:47:09      ]
19:47:09  
19:47:09 +
19:47:09  @with_input_types(T)

Done.