[v1.x] ONNX export support for multiple input data types

[waytrue17]

Description

Currently ONNX export does not support multiple input data types. It masks the first input's data type to all other input nodes. This PR enables multiple input data types, which benefits operators like where and dequantize.

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]

Hey @waytrue17 , Thanks for submitting the PR
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[waytrue17]

@mxnet-bot run ci [sanity]

[mxnet-bot]

Jenkins CI successfully triggered : [sanity]

[Zha0q1]

LGTM！