Bugfix for BackboneFinetuning + LearningRateFinder

[SkafteNicki]

What does this PR do?

Fixes #14674
Makes sure that BackboneFinetuning and LearningRateFinder can be used together.

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📚 Documentation preview 📚: https://pytorch-lightning--21224.org.readthedocs.build/en/21224/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79%. Comparing base (a7cb33a) to head (5a79452).
⚠️ Report is 6 commits behind head on master.
✅ All tests successful. No failed tests found.

❗ There is a different number of reports uploaded between BASE (a7cb33a) and HEAD (5a79452). Click for more details.

HEAD has 103 uploads less than BASE

Flag	BASE (a7cb33a)	HEAD (5a79452)
cpu	52	27
python	6	3
lightning_fabric	12	0
pytest	27	0
python3.10	12	6
lightning	29	15
python3.11	12	6
python3.12.7	16	9
python3.12	6	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #21224     +/-   ##
=========================================
- Coverage      87%      79%     -8%     
=========================================
  Files         269      266      -3     
  Lines       23708    23655     -53     
=========================================
- Hits        20536    18640   -1896     
- Misses       3172     5015   +1843