LinghtningCLI now will not allow setting a class instance as a default

[mauvilsa]

What does this PR do?

Fixes #18731

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📚 Documentation preview 📚: https://pytorch-lightning--18822.org.readthedocs.build/en/18822/

[awaelchli]

Thank you @mauvilsa, this is great!

[codecov]

Codecov Report

Merging #18822 (9b08fff) into master (c68ff64) will decrease coverage by 35%.
Report is 3 commits behind head on master.
The diff coverage is 100%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #18822      +/-   ##
==========================================
- Coverage      83%      48%     -35%     
==========================================
  Files         442      434       -8     
  Lines       35109    35017      -92     
==========================================
- Hits        29173    16880   -12293     
- Misses       5936    18137   +12201     

[carmocca]

Thank you!