Expose extract_batch_size method and add corresponding tests.

[kandluis]

What does this PR do?

This brings back this API which was made unavailable in #7882. The API is kept stand-alone rather than requiring the ResultCollection object.

Does your PR introduce any breaking changes ? If yes, please list them.

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[codecov]

Codecov Report

Merging #8357 (a519250) into master (f3e8284) will decrease coverage by 5%.
The diff coverage is 94%.

@@           Coverage Diff           @@
##           master   #8357    +/-   ##
=======================================
- Coverage      93%     88%    -5%     
=======================================
  Files         216     216            
  Lines       14097   14099     +2     
=======================================
- Hits        13068   12407   -661     
- Misses       1029    1692   +663