[MXNET-1206] Support NDArray indexing with None and Ellipsis (apache#13143)

* Support NDArray indexing with None and Ellipsis

* Update NDArray.__setitem__ docs with None and Ellipsis

* Fix boolean flag in NDArray.__getitem__, add doctests

* Add setitem test for None and Ellipsis

* Fix wrong slice used, add cases to test_indexing

* Revamp NDArray.__getitem__ and __setitem__

* Fix typo in error message of SetSliceOpOutputDimSize

* Fix setting of array with integer indices

* Fix basic __setitem__ for all test cases

* WIP: fixing advanced indexing

* REMOVE: printing in tests

* Re-implement advanced indexing with None and Ellipsis

* Fix lint errors

* WIP: fix basic indexing

* WIP: fix basic indexing

* TEMP: print statements in tests

* Fix op.slice with step<0 and end==-1

* Implement copy-free general contiguous indexing

* Improve doctest of __getitem__

* Fix missing staticmethod

* Remove superfluous _at and _slice

* Fix lint errors

* WIP: basic indexing

* Remove print statements from tests

* Fix call into op.slice in basic indexing, add doctest

* Print failing index in setitem tests

* Simplify implementation of advanced index broadcasting

* Better printing for failing setitem tests

* Remove list indexing restriction, fix value shape broadcasting

* Fix bad string formatting

* Fix bug in test_uniform

* Test mutability of sliced array if contiguous

* Fix whitespace error in matrix_op-inl.h

* "Fix" pylint complaints

* Temporarily disable failing unittests

* Silence another pylint complaint

* Fix size-0 array creation

* Make scalar tensor assignment test check for IndexError

* Re-activate operator tests with 0-size arrays

* Use np.compat in tests with zeros in shape or empty shape

* Change comment in autograd indexing test

* Add more None-containing index tuples to indexing test

* Disable None in advanced indexing test since it has not been supported

* Fix sanity

* Fix ci

* Fix unit test failure

* Fix __getitem__

Author: kohr-h