Fix cuDNN RNN dtype_with_fallback_ bug

[stu1130]

Description

#16670

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 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

@DickJC123

[DickJC123]

Please replace CUDNN_VERSION with cudnnGetVersion(). Since we only warn users if the linked-against version mismatches the compiled-against version, I think we need to consider that case here. In most cases, the user would just not receive the additional cuDNN functionality. However for the issue this PR is addressing, a user that compiled against cuDNN v7.4, but then ran with cuDNN v7.5 would subtly have their RNNs converted from pseudo-fp16 to true-fp16. Use of cudnnGetVersion() corrects this and delivers consistent pseudo-fp16 behavior. Sorry I didn't think of this when creating the original issue.

[DickJC123]

I've selectively retriggered the CI, as I don't think the recent CI failures are related to this PR. The Clojure CPU Integration error was a bit strange, so I've filed it as issue #16674 for someone to look at.

[stu1130]

Thanks @DickJC123