Unit tests for RunInference keyed/unkeyed Modelhandler and examples

[AnandInguva]

Unit tests

1. keyed examples and unkeyed Model Handler
2. Unkeyed examples and keyed model handler

These are used to catch TypeError when wrong input is passed to RunInference.

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Codecov Report

Merging #21856 (7d47ebd) into master (7bf822a) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #21856      +/-   ##
==========================================
- Coverage   74.09%   74.07%   -0.03%     
==========================================
  Files         698      698              
  Lines       92503    92524      +21     
==========================================
- Hits        68544    68541       -3     
- Misses      22704    22728      +24     
  Partials     1255     1255              

Flag	Coverage Δ
python	83.73% <ø> (-0.04%)	⬇️

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

Impacted Files	Coverage Δ
.../python/apache_beam/testing/test_stream_service.py	88.09% <0.00%> (-4.77%)	⬇️
sdks/python/apache_beam/io/source_test_utils.py	88.01% <0.00%> (-1.39%)	⬇️
...che_beam/runners/interactive/interactive_runner.py	90.06% <0.00%> (-1.33%)	⬇️
...eam/runners/portability/fn_api_runner/execution.py	92.44% <0.00%> (-0.65%)	⬇️
sdks/python/apache_beam/io/hadoopfilesystem.py	97.28% <0.00%> (ø)
...thon/apache_beam/ml/inference/pytorch_inference.py	0.00% <0.00%> (ø)
sdks/python/apache_beam/transforms/core.py	92.39% <0.00%> (+0.01%)	⬆️
.../python/apache_beam/transforms/periodicsequence.py	98.38% <0.00%> (+0.02%)	⬆️
sdks/python/apache_beam/pvalue.py	91.44% <0.00%> (+0.06%)	⬆️
...s/python/apache_beam/testing/synthetic_pipeline.py	82.29% <0.00%> (+0.24%)	⬆️
... and 1 more

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[AnandInguva]

R: @tvalentyn

[tvalentyn]

LGTM, happy to merge when tests pass.