Fix the quantization script to support Python2

[juliusshufan]

Description

The existing quantization script under https://github.com/apache/incubator-mxnet/blob/master/example/quantization/imagenet_gen_qsym_mkldnn.py can't run properly on using Python 2.7. This PR provide a fix to make it compatible with Python2.x and 3.x.
@xinyu-intel @TaoLv

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

Changes

https://github.com/apache/incubator-mxnet/blob/master/example/quantization/imagenet_gen_qsym_mkldnn.py

[TaoLv]

Do we have similar problem with imagenet_inference.py and imagenet_gen_qsym.py?

[juliusshufan]

@TaoLv Thanks for the comments, I checked the two scripts you mentioned, and indeed the imagenet_inference.py has similiar issue and modify it accordingly, imagenet_gen_qsym.py works well with Python2.
Thanks.

[TaoLv]

Thank you for the fix. LGTM.

[pengzhao-intel]

Thanks for the enhance.

[pengzhao-intel]

@TaoLv could you help merge this one?

[TaoLv]

Thank you for the contribution. Now merging.