ImageDetIter: fix property not updating bug

[zhreshold]

Description

Fix a minor property bug that label_shape is not properly updated during reshape.

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• 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 http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

[ankkhedia]

@zhreshold Thanks for your contribution!
Request you to look into CI failures

@mxnet-label-bot[pr-awaiting-review, python, test]

[kalyc]

LGTM

[stu1130]

@szha could you take a look at it?

[vandanavk]

LGTM

[vandanavk]

@mxnet-label-bot update [Python, Test, pr-awaiting-merge]

[sandeep-krishnamurthy]

data_shape is not set in the init(), should we set it?

[zhreshold]

@sandeep-krishnamurthy data_shape set in super class.