Add missing checkpoint for doctest

[amyeroberts]

What does this PR do?

The checkpoint for mobilenetv2 was accidentally removed in #21106 (see file change here), resulting in failing doctests. This adds it back.

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[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[sgugger]

Thanks for fixing!

[ydshieh]

Thank you @amyeroberts !