Fix slice op issues #13760

[mseth10]

Description

This PR fixes nd.slice for erroneous output in the following case: begin=end ( fixes #13760 )
Added unit test for the same.

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

• end=-1 with negative step is a valid input
• begin=end throws invalid input error

[mseth10]

This PR addresses the comments posted on an earlier PR #14107 (closed).

[mseth10]

Please review @reminisce @szha @anirudh2290

[karan6181]

@mxnet-label-bot add [Operator, pr-awaiting-review]

[wkcn]

Thank you for the fix! LGTM

[yuxihu]

LGTM.

[apeforest]

There are so many if/else/elseif branches in this piece of code. I know it fixes the two cases you mentioned in the description. However, instead of patching, could you please find ways to simplify the logic to fix more general cases? If not, can you add comment to the end of each branch such as // end of if (s < -1) to make the code more readable.

[mseth10]

There are so many if/else/elseif branches in this piece of code. I know it fixes the two cases you mentioned in the description. However, instead of patching, could you please find ways to simplify the logic to fix more general cases? If not, can you add comment to the end of each branch such as // end of if (s < -1) to make the code more readable.

@apeforest Simplified the logic and refactored the code. Also added comments. Please review.

[apeforest]

The change looks elegant! Thanks a lot for going the extra mile to make the code more clean and easy to read. Only one comment about unit test, otherwise it's good to ship.

[apeforest]

LGTM