Numpy indexing

[zoeygxy]

Description

• Fixed NDArray indexing: newaxis and setitem.
• Added support for numpy indexing, including basic indexing and advanced integer array indexing, None and Ellipsis.
  https://docs.scipy.org/doc/numpy/reference/arrays.indexing.html
• Added np.full operator interface; test cases not added.
• Wrapped slice_assign and slice_assign_scalar method for both numpy.ndarray and NDArray, doc added.
• Note: this PR is based on [MXNET-1206] Support NDArray indexing with None and Ellipsis #13143
• Note: the current npx.slice is not complete. Code will be updated after npx.slice [Numpy] basic slicing in symbolic interface #15663 is merged.

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)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• 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

• Fixed NDArray indexing: newaxis and setitem.
• Added support for numpy indexing, including basic and advanced indexing, None and Ellipsis.

Comments

• This PR is mainly for CI test use. Code will be merged to master branch later.
• Indexing speed issue awaiting fix.
• Need to add test cases for np.full.
• Boolean Array indexing is under construction.

[reminisce]

Only the first item of the tuple is needed in this test. Please delete all Falses.

[zoeygxy]

Fixed, thanks!

[zoeygxy]

Still fixing autograd for zero-size tensors.

[haojin2]

Closing this PR now since #15942 has been merged.