[v1.4.x] fix ssd quantization script error (#13843)

[TaoLv]

• fix ssd quantization script error

• update readme for ssd

• move quantized SSD instructions from quantization/README.md to ssd/README.md

• update ssd readme and accuracy

• update readme for SSD-vGG16

Description

(Brief description on what this PR is about)

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

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

[TaoLv]

#13843 fixed a crash issue of quantized SSD example on master branch. Since quantization is an important feature of MKL-DNN backend in 1.4.0 release, so I think this fix is critical and we should have it in the release. @srochel @sergeykolychev @pengzhao-intel

[pengzhao-intel]

I think it's important so I agree with you to cherry pick to 1.4.x

[stu1130]

Thanks for your contribution @TaoLv
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