npi.boolean_mask_assign_(scalar, tensor) operator for future boolean indexing

[haojin2]

Description

A step toward boolean indexing such as:

a[mask] = val

where mask is a boolean array and val is either a scalar value/scalar tensor/vector of values

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant JIRA issue created (except PRs with tiny changes)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at https://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$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

• Forward operator
• Unit tests

Comments

MXNET_TEST_COUNT=10000 nosetests tests/python/gpu/test_operator_gpu.py:test_npi_boolean_assign
[INFO] Setting module np/mx/python random seeds, use MXNET_MODULE_SEED=7083872 to reproduce.
.
----------------------------------------------------------------------
Ran 1 test in 187.052s

OK
MXNET_TEST_COUNT=10000 nosetests tests/python/unittest/test_numpy_op.py:test_npi_boolean_assign
.
----------------------------------------------------------------------
Ran 1 test in 115.251s

OK