Add magic method abs to NDArray and Symbol.

[kshitij12345]

Description

This magic method was missing.
Allow the natural abs(x).

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• 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

• Add abs magic method.
• Test for the same.

[wkcn]

LGTM. Thank you : )

Could you please add __abs__ for class Symbol in python/mxnet/symbol/symbol.py too? Thx!

[kshitij12345]

@wkcn Have updated the code. Please review.

[wkcn]

@kshitij12345 Great! LGTM. Thanks for your contribution!

[kshitij12345]

@wkcn The update for 1 CI check shows that it is pending, however its details show that it has completed successfully. Should I retrigger the CI or is that okay?

[wkcn]

@kshitij12345 I have retriggered it : )

[piyushghai]

@mxnet-label-bot Add [pr-awaiting-merge, NDarray]

[apeforest]

LGTM. Thanks for your contribution