make plugin type check more flexible

[jedyang97]

What does this PR do?

The current type check of plugins for Trainer is restricted to list by isinstance(plugins, list). However, if plugins is instantiated from something like Hydra or OmegaConf, it will have type <class 'omegaconf.listconfig.ListConfig'>, and the current logic would wrap that list-like object with a second-layer list and cause subsequant logic to fail. If you look at the code, it is not necessary that plugins must be a list - it suffices as long as it is iterable. Therefore, the proposed simple fix change the check from isinstance(plugins, list) to isinstance(plugins, Iterable). I hope this can benefit those who uses Hydra or OmegaConf.

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📚 Documentation preview 📚: https://pytorch-lightning--20186.org.readthedocs.build/en/20186/

[jedyang97]

Ah, I just realized that one can pass in _convert_: partial in OmegaConf to avoid casting the type to <class 'omegaconf.listconfig.ListConfig'>. In this case, I will leave it to the authors to decide whether this change is still worth it.

[lantiga]

Approved, completing the PR in #20452

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79%. Comparing base (87565cb) to head (a771423).
Report is 2 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (87565cb) and HEAD (a771423). Click for more details.

HEAD has 94 uploads less than BASE

Flag	BASE (87565cb)	HEAD (a771423)
cpu	46	24
lightning	35	18
pytest	24	0
python3.9	12	6
lightning_fabric	7	0
python3.11	12	6
python3.10	6	3
gpu	2	0
python3.12	16	9

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20186     +/-   ##
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- Coverage      88%      79%     -9%     
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  Files         267      264      -3     
  Lines       23274    23220     -54     
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- Hits        20381    18271   -2110     
- Misses       2893     4949   +2056