[WIP][DOC][v2.0] Part2: Doctest

[barry-jin]

Description

Part2 of documentation fixes. Will not be included in v2.0.0 beta release (some changes have pre-requisites of API standardization)
Add back doctest

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

Essentials

• Add shape setter
• Default float type: float64, default int type: depending on python (32-bit vs 64-bit)

Comments

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

[mxnet-bot]

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