Proposal(CLI): after_instantiate_classes hook

[AlessandroW]

What does this PR do?

Adds an after_instantiate_classes hook to the LightningCLI.
See #20400

Fixes #20400

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📚 Documentation preview 📚: https://pytorch-lightning--20401.org.readthedocs.build/en/20401/

[lantiga]

Thanks for the contribution @AlessandroW! Can you add an example for how the new hook will be used? (I'd do it in the issue directly #20400)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81%. Comparing base (c110f4f) to head (0852983).
Report is 8 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (c110f4f) and HEAD (0852983). Click for more details.

HEAD has 547 uploads less than BASE

Flag	BASE (c110f4f)	HEAD (0852983)
gpu	3	2
pytest	79	2
lightning_fabric	20	0
cpu	148	24
python3.9	38	6
python3.10	19	3
lightning	112	19
python3.11	37	6
python3.12	54	9
pytorch_lightning	19	7
pytorch2.1	27	9
pytest-full	72	24
pytorch2.3	9	3
pytorch2.2.2	9	3
pytorch2.5.1	18	6
pytorch2.4.1	9	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20401     +/-   ##
=========================================
- Coverage      88%      81%     -8%     
=========================================
  Files         267      264      -3     
  Lines       23203    23150     -53     
=========================================
- Hits        20526    18742   -1784     
- Misses       2677     4408   +1731     

---- 🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests