NPE in BucketingModule when providedLabel of DataBatch is not set

[milandesai]

Description

Using BucketingModule together with a DataBatch in which the provideLabel field is not set triggers a NullPointerException in BucketingModule.forward(...). This happens because the implicit method ListMap2Descs tries to convert ListMap[String, Shape] into IndexedSeq[DataDesc] but fails because the former is null by default. A simple fix is to do a null check in the implicit method.

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant JIRA issue created (except PRs with tiny changes)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$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

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

[milandesai]

@lupesko @nswamy

[nswamy]

@milandesai Thanks for the quick fix, @lanking520 also suggested the same fix. This will work for now, we are discussing to add new APIs(alike provideData and provideShape) to take Option[IndexedSeq[DataDesc]] in DataIter and remove the implicit def ListMap2Descs, it is also causing other problems, for example not considering the layout of the input. For RNN it is required to know whether it is the NTC or TNC format. I'll keep you posted once we have a proper solution in place aswell.

[lupesko]

@milandesai thanks for your contribution. A unit test would be great...

[nswamy]

Closing this issue in favor of #11878