fix: honor train_bn in BackboneFinetuning.freeze_before_training

[javierdejesusda]

What does this PR do?

Fixes #21531

BackboneFinetuning.freeze_before_training() called self.freeze(pl_module.backbone) without forwarding the train_bn parameter, so BatchNorm layers stayed trainable during the initial frozen phase regardless of the train_bn setting passed to the constructor.

The unfreezing path in finetune_function() already passed train_bn=self.train_bn correctly — this change makes the freezing path consistent.

Before submitting

• Was this discussed/agreed via a GitHub issue? (see BackboneFinetuning: train_bn only applied during unfreezing phase #21531)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you write any new necessary tests?
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG?

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📚 Documentation preview 📚: https://pytorch-lightning--21652.org.readthedocs.build/en/21652/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79%. Comparing base (bb7820f) to head (55c2ea2).
✅ All tests successful. No failed tests found.

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HEAD has 2053 uploads less than BASE

Flag	BASE (bb7820f)	HEAD (55c2ea2)
cpu	503	42
python	36	3
lightning_fabric	162	0
pytest	252	0
python3.13	108	9
lightning	179	15
python3.11	71	6
python3.12	144	12
python3.10	36	3
python3.12.7	108	9
pytorch2.1	36	6
pytest-full	251	42
pytorch_lightning	162	27
pytorch2.7	18	3
pytorch2.8	36	6
pytorch2.10	36	6
pytorch2.3	17	3
pytorch2.2.2	18	3
pytorch2.9	36	6
pytorch2.5.1	18	3
pytorch2.4.1	18	3
pytorch2.6	18	3

Additional details and impacted files

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  Files         270      267      -3     
  Lines       23934    23875     -59     
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- Hits        20713    18798   -1915     
- Misses       3221     5077   +1856