Significant improvement on CPU (several magnitudes of order in some cases, especially on backward pass).
Very slight improvement on GPU.

OLD MSHADOW APPROACH
--------------------

CPU
===

Timing: 50 iterations of 10 calls, shape = [1,1,28,28]
Activation Operator CPU:  Timing [Forward] 18.948 ms, avg: 0.037896 ms X 500 passes
Activation Operator CPU:  Timing [Backward] 1.658 ms, avg: 0.003316 ms X 500 passes

Timing: 50 iterations of 10 calls, shape = [1,3,28,28]
Activation Operator CPU:  Timing [Forward] 57.973 ms, avg: 0.115946 ms X 500 passes
Activation Operator CPU:  Timing [Backward] 4.748 ms, avg: 0.009496 ms X 500 passes

Timing: 50 iterations of 10 calls, shape = [50,1,18,32]
Activation Operator CPU:  Timing [Forward] 703.446 ms, avg: 1.40689 ms X 500 passes
Activation Operator CPU:  Timing [Backward] 56.255 ms, avg: 0.11251 ms X 500 passes

Timing: 50 iterations of 10 calls, shape = [50,3,18,32]
Activation Operator CPU:  Timing [Forward] 2107.77 ms, avg: 4.21554 ms X 500 passes
Activation Operator CPU:  Timing [Backward] 168.483 ms, avg: 0.336966 ms X 500 passes

Timing: 50 iterations of 10 calls, shape = [20,3,128,128]
Activation Operator CPU:  Timing [Forward] 24122.2 ms, avg: 48.2443 ms X 500 passes
Activation Operator CPU:  Timing [Backward] 1908.7 ms, avg: 3.8174 ms X 500 passes

GPU
===

Timing: 50 iterations of 10 calls, shape = [1,1,28,28]
Activation Operator GPU:  Timing [Forward] 1.637 ms, avg: 0.003274 ms X 500 passes
Activation Operator GPU:  Timing [Backward] 1.665 ms, avg: 0.00333 ms X 500 passes

Timing: 50 iterations of 10 calls, shape = [1,3,28,28]
Activation Operator GPU:  Timing [Forward] 1.562 ms, avg: 0.003124 ms X 500 passes
Activation Operator GPU:  Timing [Backward] 1.661 ms, avg: 0.003322 ms X 500 passes

Timing: 50 iterations of 10 calls, shape = [50,1,18,32]
Activation Operator GPU:  Timing [Forward] 1.635 ms, avg: 0.00327 ms X 500 passes
Activation Operator GPU:  Timing [Backward] 1.702 ms, avg: 0.003404 ms X 500 passes

Timing: 50 iterations of 10 calls, shape = [50,3,18,32]
Activation Operator GPU:  Timing [Forward] 1.83 ms, avg: 0.00366 ms X 500 passes
Activation Operator GPU:  Timing [Backward] 2.041 ms, avg: 0.004082 ms X 500 passes

Timing: 50 iterations of 10 calls, shape = [20,3,128,128]
Activation Operator GPU:  Timing [Forward] 2.08 ms, avg: 0.00416 ms X 500 passes
Activation Operator GPU:  Timing [Backward] 2.688 ms, avg: 0.005376 ms X 500 passes

NEW MXNET_OP APPROACH
---------------------

CPU
===

Timing: 50 iterations of 10 calls, shape = [1,1,28,28]
Activation Operator CPU:  Timing [Forward] 80.748 ms, avg: 0.161496 ms X 500 passes
Activation Operator CPU:  Timing [Backward] 1.176 ms, avg: 0.002352 ms X 500 passes

Timing: 50 iterations of 10 calls, shape = [1,3,28,28]
Activation Operator CPU:  Timing [Forward] 7.881 ms, avg: 0.015762 ms X 500 passes
Activation Operator CPU:  Timing [Backward] 2.181 ms, avg: 0.004362 ms X 500 passes

Timing: 50 iterations of 10 calls, shape = [50,1,18,32]
Activation Operator CPU:  Timing [Forward] 111.48 ms, avg: 0.22296 ms X 500 passes
Activation Operator CPU:  Timing [Backward] 5.408 ms, avg: 0.010816 ms X 500 passes

Timing: 50 iterations of 10 calls, shape = [50,3,18,32]
Activation Operator CPU:  Timing [Forward] 333.439 ms, avg: 0.666878 ms X 500 passes
Activation Operator CPU:  Timing [Backward] 21.331 ms, avg: 0.042662 ms X 500 passes

Timing: 50 iterations of 10 calls, shape = [20,3,128,128]
Activation Operator CPU:  Timing [Forward] 3429.19 ms, avg: 6.85837 ms X 500 passes
Activation Operator CPU:  Timing [Backward] 286.324 ms, avg: 0.572648 ms X 500 passes

GPU
===

Timing: 50 iterations of 10 calls, shape = [1,1,28,28]
Activation Operator GPU:  Timing [Forward] 1.618 ms, avg: 0.003236 ms X 500 passes
Activation Operator GPU:  Timing [Backward] 1.671 ms, avg: 0.003342 ms X 500 passes

Timing: 50 iterations of 10 calls, shape = [1,3,28,28]
Activation Operator GPU:  Timing [Forward] 1.629 ms, avg: 0.003258 ms X 500 passes
Activation Operator GPU:  Timing [Backward] 1.728 ms, avg: 0.003456 ms X 500 passes

Timing: 50 iterations of 10 calls, shape = [50,1,18,32]
Activation Operator GPU:  Timing [Forward] 1.753 ms, avg: 0.003506 ms X 500 passes
Activation Operator GPU:  Timing [Backward] 1.756 ms, avg: 0.003512 ms X 500 passes

Timing: 50 iterations of 10 calls, shape = [50,3,18,32]
Activation Operator GPU:  Timing [Forward] 1.704 ms, avg: 0.003408 ms X 500 passes
Activation Operator GPU:  Timing [Backward] 1.791 ms, avg: 0.003582 ms X 500 passes

Timing: 50 iterations of 10 calls, shape = [20,3,128,128]
Activation Operator GPU:  Timing [Forward] 2.032 ms, avg: 0.004064 ms X 500 passes
Activation Operator GPU:  Timing [Backward] 2.143 ms, avg: 0.004286 ms X 500 passes

* negative index support for sparse slice

* fix lint

* getitem(int) for csr ndarray, support a[-1]

* remove unneccessary argument

* unittest and doc update

* Final changes before RC

* Updates to NEWS.md

* Updates

* Fix Block not registering children

If the attribute was already set to something different than Block (e.g. None),
it was not being registered.

* fix if / elif for block children registration

* trigger test

* Add fix from apache#8152

* Add tests from apache#8152

* Revert "Added my code signing key (apache#8293)"

This reverts commit 22ab185.

* Revert "[CMAKE] Fix windows cmake build (apache#8227)"

This reverts commit 1c1c788.

)

* remove mshadow::range in init_op.h

* add unit test

* remove pass by ptr, add unit test for pull empty wieghts

* fix range in key partition

* remove wrong comment

* remove change for partition

* remove unused var

* add int64 to arange. add checkpointing example

* Allow test to converge

* Trigger build

* Trigger build

* Trigger build

* [Perl] emulate Python zip() for Perl

* [Perl] retool zip() uses away from the callback form

* add profile option for frontend profiling to image script

* Update image_classification.py

* Update image_classification.py

Fix a typo in the example readme.