_restricted_classmethod: add wrapper, to allow inspection

[lauritsf]

What does this PR do?

This PR resolves an issue where using inspect.getmembers on a LightningModule instance (or any class using _restricted_classmethod_impl) would incorrectly raise a TypeError. The modification introduces a wrapper in the _restricted_classmethod_impl to ensure that the method can be inspected but not called on an instance. This change maintains the intended functionality while improving compatibility with introspection tools like inspect.getmembers.

Fixes #19329

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📚 Documentation preview 📚: https://pytorch-lightning--19332.org.readthedocs.build/en/19332/

[codecov]

Codecov Report

Merging #19332 (9e2a80f) into master (d02009a) will decrease coverage by 35%.
The diff coverage is 100%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #19332      +/-   ##
==========================================
- Coverage      84%      49%     -35%     
==========================================
  Files         446      438       -8     
  Lines       37570    37418     -152     
==========================================
- Hits        31383    18292   -13091     
- Misses       6187    19126   +12939     

[awaelchli]

Looks great. Thanks for the quick fix. Would you mind adding a line in src/lightning/pytorch/CHANGELOG.md as well?