LightningModule.log_dict: fix type hints for MetricCollection

[adamjstewart]

What does this PR do?

Fixes #18641

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📚 Documentation preview 📚: https://pytorch-lightning--18646.org.readthedocs.build/en/18646/

[awaelchli]

By extension, the _log_dict_through_fabric() argument would need a small update too. LGTM thanks for the quick fix.

[codecov]

Codecov Report

Merging #18646 (350896f) into master (894952d) will decrease coverage by 35%.
Report is 4 commits behind head on master.
The diff coverage is 100%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #18646      +/-   ##
==========================================
- Coverage      85%      50%     -35%     
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  Files         427      419       -8     
  Lines       33432    33264     -168     
==========================================
- Hits        28449    16770   -11679     
- Misses       4983    16494   +11511     

[adamjstewart]

Hmm, I assume we don't want to pass MetricCollection into lightning.fabric? And we don't want to turn Mapping[str, Any] into Any and make those type hints worse... I wonder if we could make MetricCollection a subtype of Mapping...

[carmocca]

Seems like there's one more instance to update:

src/lightning/pytorch/core/module.py:611: error: Argument "metrics" to "log_dict" of "Fabric" has incompatible type "Mapping[str, Metric | Tensor | int | float] | MetricCollection"; expected "Mapping[str, Any]"  [arg-type]

[carmocca]

Ah sorry, I just saw your previous message. I would suggest that the error is ignored and you could propose making it a mapping subtype in torchmetrics in the meantime