fix test compatibility as AdamW became subclass of Adam

[haifeng-jin]

What does this PR do?

As pytorch/pytorch#143710 got merged, which made AdamW a subclass of Adam, the test test_multiple_optimizers_basefinetuning fails as it adds more param_groups to the AdamW in addition to Adam.

This PR fixes the test by enforcing a strict check on the type of the optimizer to be exactly Adam not its subclasses.

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📚 Documentation preview 📚: https://pytorch-lightning--20574.org.readthedocs.build/en/20574/

[Borda]

so this is for PT 2.6+ correct?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79%. Comparing base (ea59e40) to head (d68b2dc).

❗ There is a different number of reports uploaded between BASE (ea59e40) and HEAD (d68b2dc). Click for more details.

HEAD has 956 uploads less than BASE

Flag	BASE (ea59e40)	HEAD (d68b2dc)
cpu	238	24
lightning_fabric	31	0
pytest	122	0
python3.9	60	6
lightning	179	18
python3.10	30	3
python3.11	60	6
python3.12.7	88	9
gpu	2	0
pytorch2.1	45	9
pytest-full	118	24
pytorch2.2.2	15	3
pytorch_lightning	30	6
pytorch2.3	15	3
pytorch2.4.1	14	3
pytorch2.5.1	29	6

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20574     +/-   ##
=========================================
- Coverage      88%      79%     -9%     
=========================================
  Files         267      264      -3     
  Lines       23380    23325     -55     
=========================================
- Hits        20481    18366   -2115     
- Misses       2899     4959   +2060     

[haifeng-jin]

No, PyTorch 2.6 did not include the commit.
We use the viable/strict branch, which is ahead of PyTorch 2.6.

[lantiga]

This is looking at post-2.6 but it's good with the current releases