[mxnet 2.0] deprecate gluon.model_zoo, image.detection

[zhreshold]

Description

Deprecate gluon model zoo, image.detection in favor of gluon toolkits and gluon transform functions, respectively.

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant JIRA issue created (except PRs with tiny changes)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at https://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
• To the best of my knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

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[szha]

Thanks for bringing this up. A couple changes please:

• add deprecation warning to 1.x. the warning should only when model zoo function is invoked.
• move the current model zoo to test utils and update related tests in master.

Also, the lint needs to be fixed.

[eric-haibin-lin]

Are we going to remove the model_zoo api for mxnet 2.0? Many unit tests rely on downloading a pretrained model (e.g. quantization, AMP). Removing the API makes it harder for those tests.

[eric-haibin-lin]

I guess it's fine. The existing models in the model zoo are not trained with the numpy API anyway