[MXNET-1403] Disable numpy's writability of NDArray once it is zero-copied to MXNet

[junrushao]

Description

Per discussion with @szha , we decided that we do not allow ndarrays to be manipulated on the numpy side to ensure correctness.

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

• Set up writable flags to be False.

Comments

None

[szha]

Is there a way to completely disable read and write (since we also don't know when the read result would be accurate)? is there a way to declare that the original array no longer has the ownership of the data?

[junrushao]

@szha I haven't found a way to preventing users from reading the data. array.flags["OWNDATA"] is not allowed to be changed.

[junrushao]

@szha Hey I updated the document to reflect that we do not wish users to either read or write the numpy's ndarray if zero_copy is enabled.

[wkcn]

LGTM. Thank you! : )