Refactor RunInference api into base

[yeandy]

Moving RunInference transform from api.py into base.py.

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Codecov Report

Merging #21792 (a273b05) into master (0de9821) will increase coverage by 0.02%.
The diff coverage is 82.35%.

@@            Coverage Diff             @@
##           master   #21792      +/-   ##
==========================================
+ Coverage   74.01%   74.04%   +0.02%     
==========================================
  Files         698      698              
  Lines       92224    92337     +113     
==========================================
+ Hits        68263    68373     +110     
- Misses      22710    22713       +3     
  Partials     1251     1251              

Flag	Coverage Δ
python	83.62% <82.35%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
...examples/inference/pytorch_image_classification.py	0.00% <0.00%> (ø)
...thon/apache_beam/ml/inference/pytorch_inference.py	0.00% <0.00%> (ø)
sdks/python/apache_beam/ml/inference/__init__.py	100.00% <100.00%> (ø)
sdks/python/apache_beam/ml/inference/base.py	94.24% <100.00%> (+0.54%)	⬆️
...thon/apache_beam/ml/inference/sklearn_inference.py	92.50% <100.00%> (ø)
...eam/transforms/py_dataflow_distribution_counter.py	91.42% <0.00%> (-4.87%)	⬇️
.../python/apache_beam/testing/test_stream_service.py	88.09% <0.00%> (-4.77%)	⬇️
sdks/python/apache_beam/utils/interactive_utils.py	95.12% <0.00%> (-2.44%)	⬇️
sdks/python/apache_beam/utils/counters.py	85.39% <0.00%> (-1.36%)	⬇️
...che_beam/runners/interactive/interactive_runner.py	90.06% <0.00%> (-1.33%)	⬇️
... and 17 more

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