added mkldnn dependency for plugin compile target

[samskalicky]

Description

added mkldnn dependency for "plugin" compile target in Makefile. Without the change, building fails following warpctc instructions here:
https://github.com/apache/incubator-mxnet/tree/master/example/ctc#ctc-loss-in-mxnet

Fixes #14236

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

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

[apeforest]

Why do we need warpctc if mxnet already has native implementation?

[TaoLv]

@xinyu-intel Could you help to check if this change fixes the issue?
@samskalicky @marcoabreu Is it possible to add the build to CI?

[xinyu-intel]

@TaoLv Build successfully on my local ubuntu environment.

[samskalicky]

Thanks @xinyu-intel for the testing!

@apeforest I think @szha answered your question on the dev list: https://lists.apache.org/thread.html/5264c271ea1f87bf747c8fa44cece82df966fc950ab920c5f109f0de@%3Cdev.mxnet.apache.org%3E . We should discuss deprecating warpctc specifically on the dev list.

It seems that warpctc does more than just ctc loss, it seems like it also does grouping/sorting. There is an example of removing warp ctc and just grouping the features manually: https://github.com/Xilinx/mxnet/blob/master/example/warpctc/lstm.py#L114. So basically group manually, rather than use warpCTC "loss" to do it. So might not be a 1-to-1 replacement with the ctc_loss operator.

[anirudhacharya]

@mxnet-label-bot add [pr-awaiting-review]