[v1.x] Backport the GroupNorm implementation fix and introduce a migration path

[nickguletskii]

Description

This is a backport of #18199 for MXNet 1.x that doesn't break compatibility with existing models.

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

• Add a switch that switches between the old and the new implementation.
• Update the the unit test to test both implementations.
• Document the operator's switch.
• Document the switch in the Gluon block.
• Add a warning in the Gluon interface.

Comments

• Since GroupNorm will be moved to npx in MXNet 2.0, introducing a (dummy) v2 parameter in MXNet 2.0 is not necessary, since backward compatibility will be broken anyways.
• Should this be included with MXNet 1.7?

[mxnet-bot]

Hey @nickguletskii , Thanks for submitting the PR
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[nickguletskii]

The linter checks are currently failing due to false positives in unrelated files: #18269

[nickguletskii]

@mxnet-bot run ci [edge, unix-gpu]

[mxnet-bot]

Jenkins CI successfully triggered : [unix-gpu, edge]

[nickguletskii]

@mxnet-bot run ci [all]

[mxnet-bot]

Jenkins CI successfully triggered : [windows-cpu, unix-cpu, miscellaneous, centos-gpu, unix-gpu, clang, windows-gpu, sanity, centos-cpu, website, edge]