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 (0c2025d) to head (267371d).
✅ All tests successful. No failed tests found.

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HEAD has 726 uploads less than BASE

Flag	BASE (0c2025d)	HEAD (267371d)
cpu	198	33
python	18	3
lightning_fabric	54	0
pytest	99	0
python3.12	54	9
python3.13	18	3
lightning	90	15
python3.11	36	6
python3.12.7	54	9
python3.10	18	3
pytorch2.8	18	6
pytorch_lightning	54	18
pytest-full	99	33
pytorch2.5.1	9	3
pytorch2.7	9	3
pytorch2.1	18	6
pytorch2.3	9	3
pytorch2.2.2	9	3
pytorch2.6	9	3
pytorch2.4.1	9	3
pytorch2.9	9	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    18965   -1902     
- Misses       3204     5056   +1852