Check expected vs. actual Node inputs. Many operators non-compliant.

[DickJC123]

Description

PR #15688 fixed, among other things, a discrepancy between the number of inputs specified in the _backward_clip operator definition via .set_num_inputs() and the number of inputs actually created for the backward node (1 vs 2, respectively). Use of monitors (for debugging) requires these input counts to be equal.

This PR supplies the check of expected vs. actual node input counts and exposes the fact that many operators have a discrepancy. As suggested by @ptrendx, this check is best supplied in this separate PR, as it might take some time to fix all the non-compliant operators. The first commit only supplies the check, not operator fixes, and so is expected to fail.

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

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

• 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

[marcoabreu]

Installation exceeds the scope of this script IMO

[ptrendx]

Relevant changes from this PR were incorporated in #17049, so closing this one.