[v1.x] ONNX export support for RNN

[waytrue17]

Description

ONNX export support for RNN. Currently it only implements a subset of attribute combinations to support a customer use case.

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

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

[mxnet-bot]

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[Zha0q1]

Thanks for the speedy implementation! Left two very minor comments

[Zha0q1]

LGTM thanks!