Add CustomOp tutorial doc

[rondogency]

Description

Add a brief tutorial doc on CustomOp tutorial and examples

This PR is not

• Updating docs on the MXNet website on extending MXNet
• Consolidating operator related docs on the website

Those will be addressed in the coming PRs

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 best of my knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

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

[rondogency]

@samskalicky @mseth10 @wkcn please take a look at the doc

[samskalicky]

Good first draft!

[rondogency]

@samskalicky @wkcn resolved all the comments!

[rondogency]

@eric-haibin-lin @aaronmarkham can you also take a quick look at the doc, thanks!

[samskalicky]

LGTM, great job @rondogency

[wkcn]

Good job! LGTM : )

[eric-haibin-lin]

can we update the doc on website on extending mxnet with ops? https://mxnet.apache.org/api
We need to probably update https://mxnet.apache.org/api/faq/new_op with a new section for extending mxnet with an op without rebuilding mxnet.

[eric-haibin-lin]

There are multiple docs on writing operators. Shall we consolidate them?

• python op: https://github.com/apache/incubator-mxnet/tree/3ece00b1acf3e5ca2bbf46d6eaf36ae900cd7666/example/numpy-ops
• cpp op guide: https://mxnet.apache.org/api/faq/add_op_in_backend
• old cpp op & python op guide: https://mxnet.apache.org/api/faq/new_op (it mentions mshadow, which is supposed to be deprecated...?)
• this tutorial

[rondogency]

@eric-haibin-lin Thanks for the review! I will add it to a TODO list with upcoming features and consolidating all docs about writing operators (to give new users a smooth experience)

[aaronmarkham]

How do I go from running this basic example to consuming the following info for my own op? Maybe even a simple example of customization for a particular use case would help.

[rondogency]

@mxnet-label-bot add [pr-awaiting-review]

[aaronmarkham]

Super minor suggestions, not worth another CI run. But, if you have to do it anyways, then please add these minor changes.

[rondogency]

@aaronmarkham thanks for the approval! I resolved your comments since I have to add license header anyway.

[samskalicky]

@mxnet-label-bot update [pr-awaiting-merge]