Bugfix/18394 batch size finder max val batches

[BoringDonut]

What does this PR do?

Resets the CombinedLoader attribute of the eval loop to trigger a re-initialization with the correct val_limit batches.

Fixes #18394

(Adapted from #17636)

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📚 Documentation preview 📚: https://pytorch-lightning--18854.org.readthedocs.build/en/18854/

[awaelchli]

Thanks @BoringDonut, good call to carry over the same fix from the LR finder. Could you also add a changelog entry in the bugfix section?

[codecov]

Codecov Report

Merging #18854 (75c218b) into master (8748258) will decrease coverage by 28%.
The diff coverage is 100%.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #18854     +/-   ##
=========================================
- Coverage      76%      48%    -28%     
=========================================
  Files         443      435      -8     
  Lines       35374    35224    -150     
=========================================
- Hits        26708    16894   -9814     
- Misses       8666    18330   +9664     

[BoringDonut]

Thanks @BoringDonut, good call to carry over the same fix from the LR finder. Could you also add a changelog entry in the bugfix section?

@awaelchli added a changelog entry ~