avoid skipping validation stage when data read iterator exists #18289

[szha]

Description

#18289 for v1.x

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 @szha , Thanks for submitting the PR
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[szha]

I reviewed this PR in #18289. Thanks to @heaseny for the original contribution.