Loading train_dataloader before estimating max_batches

[shihchengli]

What does this PR do?

Fixes #16060

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📚 Documentation preview 📚: https://pytorch-lightning--20148.org.readthedocs.build/en/20148/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81%. Comparing base (345450b) to head (af3648f).
Report is 4 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (345450b) and HEAD (af3648f). Click for more details.

HEAD has 87 uploads less than BASE

Flag	BASE (345450b)	HEAD (af3648f)
gpu	4	2
pytest	25	2
lightning_fabric	7	0
lightning	31	16
cpu	41	21
python3.9	12	6
python3.10	12	6
python3.11	12	6
python3.12	5	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20148     +/-   ##
=========================================
- Coverage      89%      81%     -8%     
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  Files         267      264      -3     
  Lines       23071    23019     -52     
=========================================
- Hits        20572    18611   -1961     
- Misses       2499     4408   +1909