[FEATURE] Put every class in training set

[mjamroz]

Description

To make sure training set image list contains at least one image of each class.Having all images of some class in val set makes a mess doing inference with resulting model (and synset).

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 @mjamroz , Thanks for submitting the PR
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[mxnet-bot]

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[mjamroz]

@mxnet-bot run ci miscellaneous, unix-gpu, unix-cpu, website

[mxnet-bot]

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