Fix dataloader not reloading when resuming from checkpoint

[littlebullGit]

When resuming from a checkpoint with reload_dataloaders_every_n_epochs, the dataloader was not being reloaded at the correct epoch. This was because setup_data() was overwriting _last_train_dl_reload_epoch with the current epoch during checkpoint restoration, losing the information about when the dataloader was actually last reloaded.

The fix:

1. Save _last_train_dl_reload_epoch in checkpoint state
2. Restore _last_train_dl_reload_epoch from checkpoint on load
3. Only update _last_train_dl_reload_epoch when actually reloading the dataloader or during initial setup (not when resuming)

This ensures _should_reload_train_dl returns the correct value after resuming from a checkpoint.

Backward compatible: old checkpoints without this key will default to float('-inf'), which triggers a reload (the safest behavior).

Fixes #21492

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📚 Documentation preview 📚: https://pytorch-lightning--21514.org.readthedocs.build/en/21514/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79%. Comparing base (7d2de87) to head (c656110).
✅ All tests successful. No failed tests found.

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HEAD has 345 uploads less than BASE

Flag	BASE (7d2de87)	HEAD (c656110)
cpu	115	33
python	11	3
lightning_fabric	35	0
pytest	65	0
python3.10	12	3
lightning	55	15
python3.13	10	3
python3.12	29	9
python3.11	23	6
python3.12.7	30	9
pytorch2.1	11	6
pytest-full	50	33
pytorch2.3	6	3
pytorch2.5.1	4	3
pytorch2.2.2	5	3
pytorch2.8	9	6
pytorch_lightning	25	18
pytorch2.9	4	3
pytorch2.6	4	3
pytorch2.4.1	4	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #21514     +/-   ##
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- Coverage      87%      79%     -8%     
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  Files         270      267      -3     
  Lines       24071    24021     -50     
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- Hits        20867    18968   -1899     
- Misses       3204     5053   +1849