[numpy] Cosmetic improvement on mxnet.numpy builtin op signature in documentation

[reminisce]

Description

There is a subset of operators exposed to Python frontend with MXNet auto-generated APIs. This had resulted in inconsistent signature appearance in the documentation with the official NumPy package which is not desirable. For example:

>>> from mxnet import np
>>> help(np.sum)
sum(a=None, axis=_Null, dtype=_Null, keepdims=_Null, initial=_Null, out=None, name=None, **kwargs)

This PR has addressed this issue by replacing those ops' __signature__ field using the signatures from the documentation file: _numpy_op_doc.py. For example:

>>> from mxnet import np
>>> help(np.sum)
sum(a, axis=None, dtype=None, keepdims=False, initial=None, out=None)

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

[reminisce]

@leezu Could you help to review this if you have time? Thanks.

[haojin2]

LGTM