[MXNET-313] Python: Resolve some of the undefined names

[cclauss]

Description

(Brief description on what this PR is about)

• Partial fix to the undefined names discussed in 5 undefined names in Python code #8270
• When imports got confusing, isort was run to sort the imports
• Use feature detection instead of version detection on imports

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

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

[marcoabreu]

Thanks a lot for fixing the examples!

Committers, I think we definitely need to require tests for all examples since we're constantly running into issues like this.

[cclauss]

$ flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics

In Travis CI would be a nice step forward.

[ThomasDelteil]

nd has been imported as ndarray so you could use ndarray.NDArray I think to remain consistent with how it's been done in this file

[ThomasDelteil]

C: 28, 0: third party import "import numpy as np" should be placed before "import mxnet as mx" (wrong-import-order)

• could you please create a JIRA ticket for your PR? Thanks!