move choose_element_0index to operator

[roywei]

Description

fix #7853
move legacy choose_element_0index as an alias of pick operator.
It's still used in Scala, CPP package and DQN example.

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

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

[roywei]

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

[ChaiBapchya]

LGTM!

[wkcn]

Thanks for your contribution!
LGTM: )

[wkcn]

Will the document of choose_element_0index be the same as that of pick?

I think it is better to abandon choose_element_0index, and let other related projects use pick operator.