Fix typehint for WandbLogger's log_model argument

[SebastianGer]

What does this PR do?

Fixes #18370

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📚 Documentation preview 📚: https://pytorch-lightning--18458.org.readthedocs.build/en/18458/

[SebastianGer]

I added a test that fails before the bug fix and clears afterwards. I assume it should contain some kind of mock.patch for WandB, because that's included in the other WandbLogger tests. But I don't understand enough about this to know what to patch in where.

@awaelchli You seem to have worked on several commits of test_wandb.py, could you help with that?

I'm also not sure if the test should be moved somewhere else, but I guess that's something that can be looked at during the final review.

[codecov]

Codecov Report

Merging #18458 (8c00623) into master (41ba7de) will decrease coverage by 34%.
Report is 2 commits behind head on master.
The diff coverage is 100%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #18458      +/-   ##
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- Coverage      85%      50%     -34%     
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  Files         427      419       -8     
  Lines       33263    33131     -132     
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- Hits        28119    16601   -11518     
- Misses       5144    16530   +11386