fix: coco category ids can now be not sequential - avoiding memory leak

[patrontheo]

Closes #4293.

What changes are proposed in this pull request?

parse_coco_categories returns classes_map instead of classes. It is a dict mapping class IDs to class labels. In areas where the list of classes are needed, it it obtained through list(classes_map.values()).

The caveat I see here is that the public facing add_coco_labels function takes classes as an argument (the list of class labels). To avoid changing the public interface, I did not modify that and built classes_mapusing classes_map = {i: label for i, label in enumerate(classes)}. This should work but requires that in labels_or_path files, the category IDs are sequential (starting from zero). This was already the case before this change.
If we want to allow non-sequential category IDs here also, we need to change the public interface and require classes_mapinstead of classes. Or do you see another solution ? Should we change the public interface or leave it like that ?

How is this patch tested? If it is not, please explain why.

Not thoroughly. I am opening a draft PR to run the tests, as explained in Contributing.

Release Notes

Is this a user-facing change that should be mentioned in the release notes?

• No. You can skip the rest of this section.
• Yes. Give a description of this change to be included in the release
  notes for FiftyOne users.

(Details in 1-2 sentences. You can just refer to another PR with a description
if this PR is part of a larger change.)

What areas of FiftyOne does this PR affect?

• App: FiftyOne application changes
• Build: Build and test infrastructure changes
• Core: Core fiftyone Python library changes
• Documentation: FiftyOne documentation changes
• Other

Summary by CodeRabbit

• New Features
  • Enhanced label handling in COCO dataset utilities by introducing a mapping of class IDs to labels for improved accuracy and flexibility.

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Walkthrough

The update in the fiftyone/utils/coco.py file introduces a classes_map to efficiently map class IDs to their corresponding labels. This mapping replaces direct references to classes across various functions, enhancing the handling and flexibility of class identification in COCO dataset operations.

Changes

File Path	Change Summary
.../utils/coco.py	Introduced classes_map for mapping class IDs to labels, replacing direct references to classes in multiple functions. Adjustments made to handle classes based on classes_map presence.

🐰✨📜
In the land of code, where the rabbits hop,
A new map was drawn, and old paths do stop.
classes_map now leads the way,
Through COCO fields, where data play.
Cheers to changes, both big and small,
Hop along, coders, one and all! 🌟🥕

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[brimoor]

@patrontheo thanks for taking this on! I'll defer to @swheaton for functional review.

With respect to your comment about add_coco_labels(), perhaps you could update so that its classes argument can be either a list (current) or a dict (new format)?

[patrontheo]

With respect to your comment about add_coco_labels(), perhaps you could update so that its classes argument can be either a list (current) or a dict (new format)?

Yes, good idea, I did that.

For the Exporter, I haven't tried yet to use existing IDs as you suggested (in #4293), but I'll see if I get a bit of time to look at it.

[patrontheo]

@swheaton any input ?

[brimoor]

LGTM, I'll handle my suggestion to retain class mappings when exporting in a follow-up commit 👍

[swheaton]

@patrontheo sorry been very busy, prepping to go on leave.

Thank you for the contribution!!